Bitcoin was the first ever crypto and as they say in business first to market is a huge advantage. It has been a consistent performer year over year (except this year of course) and is what most people believe crypto is all about

It is also the most widely accepted coin and has had major businesses such as Dell, Microsoft, Steam, etc all accept it at one point or another. That is a huge reason right there it dominates the market. People believe it has value for more than just drugs if places like Microsoft will accept it.

The value is another reason. People perceive value based on price of something, with bitcoin being so high vs others it MUST be the right coin to get into.