Hmmm because our primary target (middle aged women) are not that web-savvy i guess, and the .com is the default TLD. We bagged sleep.io when it first went on sale but haven't considered using it as the default.

To ask the opposite (and possibly stupid) question - why would you make sleep.io the default?

Somebody was in the news the other day because their clever country-code TLD was taken away by the registry. All of the country code TLDs have weird restrictions with which you can easily fail to comply. Recourse for smaller registries can be harder to obtain.

In this case I'm not sure why they went with 'sleepio' if they weren't going to use '.io' other than perhaps it's trendy right now, but I think they probably made the right move getting the .com.

Answered a similar question the other day. Webmaster tools doesn't let you target .io domains globally, as it's intended for the Indian Ocean countries. Therefore it won't rank as well as a .com in the Google serps, which is a big disadvantage.