Stripe's amazing service also comes at a substantial premium. For a high-volume thin-margin company like Lyft where every penny counts, per transaction fees at 2.9%+ are killers. Furthermore, large companies prefer to own the raw card data themselves as the interchange rates for card not present debit cards is different than card not present credit cards. Once you hit a certain amount of scale this .15% difference is substantial.

(edit: I missed that parent comment was about trust in a 3-person startup, but point is still somewhat valid)