Probably because OTR only works when both parties are online at the same time.

Edit: that said I haven't looked at their solution. Maybe it has the same issues. Or worse.

Looking at their crypto code, it appears they assume both parties are online. There is a two way hand shake for key negotiation.

My guess was they wanted to handle things like video chat and file transfer that OTR doesn't handle. But at least for video chat, I don't think it NACL will work out of the box either