Because they are cool! Also identifies something as having developers or technical folk as the target audience. And because of it's popularity, it's one of the few generally trusted top-level domains (by both humans and mail daemons)

For me personally, it came down to the fact that $firstname.io was available, but .com, .org, .net, and whatever else I checked wasn't. Kind of regret it now, not so much for the price but because of the questionable political situation with the continued availability of the TLD doing forward.

I/O is used frequently in tech as an acronym for Input/Output. I suppose that's why it's popular for techy stuff like SaaS providers and such. It's also a moon of Jupiter which adds to the nerdgasm.