Because crypto is mostly speculative with no actual legitimate usecase, and really (except for speculative NFT wielding crypto bros) nobody outside of the cryptosphere (i.e. normal people) uses it.

There are only downsides with crypto and that is incredibly risky for Stripe as they don't want to be a facilitator of scams, NSFW payments, ransomware and other illicit activity that crypto is associated with.

It is simply not worth the risk.