@OP it is quite simple because there are people willing to compete with the price of Bitcoin just to own it.  We all know that in a free market, price is dictated by the demand.  If the price in continuously increasing, it simply means that there are more and more people willing to spend their money to have Bitcoin.

The reason behind why people wanted to acquire Bitcoin varies from person to person but most of them are hoping that they will get a huge profit from holding it for years and sell it at their target price.  Others find Bitcoin the idea that holding it makes them control their own wealth far from the prying eyes of the government and so on.

Well there are quite lots of reason why people are investing in Bitcoin rather than Altcoins
And if it's for quick profit many would flock to altcoins.
Bitcoin was created because the current traditional system is failing
And history can back the success of Bitcoin.
Most popular is that is a means of accumulating wealth to to mention it been an hedge over inflation.


In summary It's trust, Bitcoin has given adopters reasons to trust it over others.

Bitcoin adoption growing fast in relative to altcoins would be better
You don't subscribe to Bitcoin, You own Bitcoin.

One of the reasons people invest in Bitcoin rather than shitty altcoins despite its high market capitalization is because it's more safer to invest than altcoins. Some altcoins after launching may drop in price and not recover back or the dev may rug the community, leaving investors in losses. Bitcoin on the other hand has been around for a some time, it's community owned, has a proven track record and no matter how deep it falls in price, it will bounce back.

Bitcoin is all that matters in the long time. All other cryptocurrencies are generally referred to as altcoins because none of them can be able to stand the position Bitcoin is today and their existence has a a stronghold of what bitcoin is today. Bitcoin was the first cryptocurrency and till date its decentralized nature has made it a more dependable coin to invest in rather than depending on some altcoins that the future of them may not be certain and has a very high probability of not staying long in the market. Telling them that is how bitcoin started has been the case of many altcoins and several years down the line, they’re either no more in the market or have lost their value and cannot grow higher than what they are today. The adoption and use case of bitcoin will make it a more dependable option to go for in the midst of many coins that their value can no longer be dependent on.

Bitcoin's price is expensive because it has a limited supply of 21 million Bitcoins, from which 19.8 million coins are mined and 3.8 million coins are lost. Bitcoin is probably the only decentralized coin because it's the most adopted coin, mined all around the world and it's creator is unknown for the world, he remained anonymous and disappeared, which strengthened the decentralized legacy of Bitcoin.

So, there is a limited supply and there is a demand, many institutions are adopting Bitcoin and there are still many more that will adapt Bitcoin because it's the future, it's simply inevitable. Bitcoin ETF approvals prove again that Bitcoin will further be adopted. So, that's why people invest in it.

Most of the altcoins now have been pumped already. And whenever someone says that Bitcoin was already very expensive, it will always be with that kind of description even in bear markets. You're right, people keeps on having a double mind to invest because they're doubting it even when the prices are considered low.

Change your title, Bitcoin has no membership. It's a decentralized cryptocurrency and a store of value for which we are all holding it as an asset. Maybe Bitcoin's "popularity" is rapidly growing.

Most altcoins have a lot of problems when compared to Bitcoin:

1) they are not really decentralized, they are managed by a single company -- this basically means they don't really need a blockchain and make no sense
2) if they are PoW currencies, they are far less secure than Bitcoin,
3) if they are PoS currencies, their security depends on a certain degree of centralization,
4) many have only speculative usage (with a few exceptions like ETH, XMR, SOL, LTC ...)
5) from those promising some sort of "new paradigm", most of those "paradigms" are small improvements which could often be adopted by other altcoins too,

In addition,

- altcoins have historically underperformed Bitcoin in all timeframes longer than 5 years,
- there were lots of scams in the altcoin sector, scaring possible users away (and often the users are correct ..)
- Bitcoin got ETFs, only two other altcoins have ETFs (ETH and SOL, SOL only in Brazil)

This is why Bitcoin's "membership" and also its market cap in comparison to altcoins -- popularly called "Bitcoin Dominance" -- is growing, see here.

Just a little addition: While "membership" may look strange and seem not totally accurate, I don't think it's totally wrong, because users of a cryptocurrency do form part of some kind of "decentralized organization" (DAO) or group.

Back in the past on which it isnt really that getting that much attention or recognition but still there are still those people who are really that believing into this technology and thats why the had risked out on investing into those moments or time that it was still that still cheap. So how it do end up? They are the ones who do make money with this current market condition specially on breaking up new all time highs.
No one had anticipated that this would really be that become too big on which it did really comes into a point that government and institutions are really that making up some recognition and now that it is really that starting these funds are flowing into this crypto market which does simply means that it is really that making some noise now and even government and billionaires are really that been starting out to dive in.

Adoption and recognition is on the move and one of the most notable event is that when the time that El Salvador did make Bitcoin as legal tender on which it did really shook up other countries on such decision.
They did even accumulate even more on the time that Bitcoins price dip on $15-20k as far as i remember and that one really paid off if we do base up on the current price condition that we are into.
This is why if you are an investor then riding up with the waves will really be the best thing to do.

I don't want to say that I am a member because indeed I am not a member but when it comes to bitcoin I would be very confident that I am one of the people who own bitcoin or the current owner of bitcoin.
The reason why I am in bitcoin is ultimately about confidence and seeing the potential of what has happened since a few years from when they existed until now so by seeing good progress like this then it is clear that being in bitcoin is a very worthy thing to do.

Why not with altcoins? actually this is the choice of each and I think everyone must have their own thoughts on this matter but for me personally as long as I can be in bitcoin and still think bitcoin is the king then why do I try in altcoins which are indeed in terms of conditions and management the risk is clearly much greater.