I'm just speculating (please chime in if you know more!). Afaiu protocol buffers are very popular for over the wire communication and in that use case cap'n pr

I'm just speculating (please chime in if you know more!). Afaiu protocol buffers are very popular for over the wire communication and in that use case cap'n proto does not win you much @ performance (?). And if you don't go over the wire the performance difference might not matter for many use cases (e.g there is plenty of json sent around as well) and when it does matter: cap'n proto or something custom w/ desired performance characteristics other than protocol buffers is chosen but here cap'n proto covers a slim middleground (?). + usual adoption dynamics: protocol buffers have more mindshare and if another solution is not clearly way better for the use case: switching/trialing is not happening as much. (?)