The thing is that they both target similar applications. Zipkin has been around for years, and as both support sending data to it, surely there's enough common ground. I understand what you mean regarding infra, but I think the parties could work towards unity in future.

gRPC supports OpenCensus for example, I recall that OpenTracing has been working on an RPC for reporting traces. gRPC by being the RPC framework, it would make sense if it could self-report to OT in addition to OC.