Because altcoin is very convenient for investment because the cost is easy and can invest with small capital and easy to get profit with short and long term, bitcoin now very high fee and for the players of small capital will run out of funds to pay fee. And we can directly participate in interesting projects without intermediaries and have payment channels that are independent of banks.
Bitcoin does not have a high fee at the moment, i have just sent a bitcoin transaction of a few hundred dollars and it only costed me less than $1 for sending that. So yes, for me, altcoins are more than needed, but investing in them depends on every person.