I think the reason is when investing in altcoins, the pays off we get is much faster than any other, if you have known what to do. It is sometimes a short-term and medium-term prospect.
I would also consider on the long term, I actually believe as long as you choose the right one this would give you a better future.
Bitcoin is considerably one of the best investment in crypto but altcoins should not be taken for granted as they can also grow and have the success like bitcoin.