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 agree, bitoin popularity reached to different places and niches, bringing more investors to adopt bitcoin and some gangsters too (inevitable they wouldn't adopt bitcoin). Bitcoin has more trust than any other crypto currency.
I ask myself if another crypto currency were the first one to be created, bitcoin would have the trust it has today...