Imho the main reason - Bitcoin is the most famous cryptocurrency. Even people which are far from all of this blockchain stuff, know what is it. It's maybe not the most perpective or workable, but it's the face of all cryptocurrency-sphere.
I am not so sure that his main power is in his fame. I am actually seeing a lot of speculation in the current "power" of bitcoin, and I would not be surprised if this inflationary bubble ends up bursting sooner or later.