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I'd have to argue it's because Bitcoin went net-positive after the BCH fork -- that shows a bit of resilience and probably would inspire false confidence in speculators.

First mover advantage. The Bitcoin paper and idea are world famous and while nowadays everyone has read/heard and is talking about Bitcoin, almost no one knows about other cryptocurrencies. Also I suppose the community is much larger and there are more miners.

Popularity, and all that naturally follows.