[pjc50]

People think serializers are easy, when suddenly this makes a whole raft of choices your problem. You can write your own, and now you have two problems; or you can build on top of capnproto or grpc or, if you really want to get retro, ASN.1 and inherit the problems of your serializer framework.

JSON by comparison has simple, obvious limitations that more people are familiar with dealing with.

There's also the tendency to tie your protocol to implementation. The Microsoft Exchange "protocol" didn't get reverse engineered for so long because it's basically the COM structure of Outlook fed through (if I remember rightly) DCOM-RPC.