The most logical reason for that is; Bitcoin was created first and get the hype in the early stage. Altcoins tech is better and they have more features but whatever they do, nothing can replace Bitcoin. Bitcoin is a revolution.
Being the first or Alpha does not ensure a spot to be the "king" as most would say. It is true that there are some altcoins that works better that Bitcoin but still, no one replaces Bitcoin. The mere explanation for this is the people support. People support bitcoin more than any cryptocurrency out there. Without this support, no altcoin could dethrone Bitcoin.