Because it hasn't been dominated by domain squatters and it has a techie association (input-output).

But I still wouldn't use it for a serious business venture. Look at how a lot of previous x.io companies have now switched to xio.com.

The trend appears to be starting out on .io and then when you get enough traction / money, buying the .com which would previously have been too expensive. I don't think many of the .io companies would consider their original domain a mistake.