Simple, Millennials like technology rather traditional investment such as gold. They are aggressive too in terms of profit so its obvious that many millennial will prefer BTC over Gold since its a high risk high reward investment. Technology is evolving and people adapt on it so its not a shocking stats.

I don't know the reason why this issue is a bad thing to BTC??

Which isn't a problem. Regardless what percentage of bitcoin are locked up vs. being transacted with, what matters is that Bitcoin is being used both as a store-of-value and as a payments currency. People are going to use Bitcoin for whatever purpose they want to, and one of those is purposes is for SoV. We can't force people to use Bitcoin for a specific purpose(payments) just because it says "A Peer-to-Peer Electronic Cash System" on the whitepaper's title.

I would say Gold or other similar commodity are a hedge against inflation. It would make sense for the more conservative population to be purchasing precious metals as compared to other commodities. I wouldn't say that it for certain but most of the younger generation tends to be more well versed in technology and are generally more willing to take the risks. I find Bitcoin to be more of a suitable investment as compared to Gold and if history is any indication, Bitcoin tends to fare well in the long term.

Probably bitcoin is an old fashion of way in investing and we  can have the new way as investing like digital currency in which the value of earning profit could be more than gold. This is why the new generation are after and usually can adapt the new trend of this technology. Traditional investor probably wanted to join in the bitcoin investment but it could be a hindrance to them in where they needed to learn the technology first in which they do not have in their time.

Because millennials prefer to invest in they think they can profit the most compare to traditional gold investment which is complicated hard to buy and really hard store if you want to keep it by yourself, crypto is very easy to managed,buy and sell and exchange to fiat, Im going into bitcoin also over gold. 

It is due to the fact that Millennials are easily attracted when they see an asset that is very profitable and have a huge price in the market. These days, if you ask someone what is bitcoin, they probably have an idea already so they will answer immediately. But still gold is much more popular as an asset compared to bitcoin because if you ask some indigenous people in a certain are, they know what gold is, so they will probably choose gold to invest rather than bitcoin. Also bitcoin is an active source of profit while gold is just a passive asset that you need to wait for so many years.

A couple of weeks ago, we saw another conclusion base on a customer survey from deVere being extrapolated over an entire population of Millionaires (see re: Millionaires FOMO: 73% Will Own Bitcoin by 2022, Survey ).

The deVere survey on Milennials focuses on over 700 millennial clients, as per their own wording. That means that, if the survey group is well selected and distributed, the conclusion may be right in the context of their own customers, and perhaps that of customers of similar corporations.

What would be interesting is to perform a street level survey in different whereabouts, and then compare the results, as well as seeing the amount who answer that they haven’t got a clue. Customers of a financial consultancy such as deVere are going to be more alike and investment versed than a similar group randomly chosen at street level, thus what the survey concludes may not be applicable to the general millennial population.

For my understanding they find it much easier thn the gold since Millennials are more interested in the internet and technology.
They might also see it as an opportunity to have an investment since most of them only see's gold as a jewelry and not as an investment.

Millennials are most likely to buying BTC and other cryptocurrencies like MintMe since as someone already said, it's an easier asset to manage by yourself with no trouble.

Millenials are more likely to buy bitcoin than gold for the same reason why I do.

I don't know where to buy physical gold safely. I also read that you have to pay premium if you want to buy physical gold compared to average gold prices on the market. These prices are based on gold shares. Physical gold which would be the only one for me is more expensive so not really worth it. With Bitcoin there's no real and unreal unless you hold it on exchanges then it's really not your bitcoin just like gold in shares is not your gold.