There's more to it. Some of these arguments have been mentioned by Bob, but I'll put my main arguments here:
I and O key are situated together on most common keyboards configurations, making it fast and easy to type, especially on mobile devices.
IO is shorter than COM. Both of these matter when browsing on mobile devices.
Google considers it to be a gTLD.
IO is more affordable than COM but still carries legitimate weight in tech & gaming branche, which are huge.
Strong association with cryptocurrency, which is bound to have an increasing influence on financial systems.
Then there's popular business networks like apollo.io. Serious and legitimate brands operate on this TLD.
Expensive reg fee means less abuse.
Any software developer will likely visit a .IO domain multiple times a day, since many popular libraries and tools are hosted on IO domains. ( libraries.io, socket.io, jenkins.io, material.io, etc )
As explained in my other thread, most kids & teenagers play or have played .IO games at some point.
Many popular casual games are associated with the .IO extension, so there is a level of familiarity with the domain that will carry over into future generations.
I mean, there's a lot of optimistic speculation going on at this point, but imo it does seem warranted.