Bitcoin is the safest investment, but not directly the best in terms of cold hard % returns, especially in the short term. People figured out that Bitcoin will be going up anyway, and that altcoins follow at a much faster rate (it's almost like they come with a free leverage multiplier), which is why they prefer to park the little money they have in altcoins.
I can't even blame them for utilizing the funds they have in the most efficient and profitable way. The majority of the people are here only for the profits and not the fundamentals making Bitcoin such a powerhouse. That's why I am happy to have the option on Bitmex to use my BTC to speculate on altcoins. By the time the bull run takes off, my BTC position in altcoins will pay off big time, and all in BTC.