Ask HN: Why are lots of startups using the .io domain?

I guess,

1 - It implies IT tech related, IO = input/output, as pointed out by otoburb;

2 - Domain name availability, as pointed out by yolesaber;

3 - Google named its developer conference 'Google IO', just a guess.

This is the correct answer.

Appeals to the HN audience obsession with technology, due to the similarity and oblique reference to input/output (i/o).

Domain name availability.

Agreed. Domain name availability is very hard sometimes. For others it might just make sense. Example: if I created a webapp and for some reason wanted to call it Sendio, sendio.com is taken but I can always get send.io and run my app from there with a cool url

I'm not sure, but the only one that makes sense so far is filepicker.io

firehose.io