Its maturity is not that interesting. I worked with Sun XDR back in the day (roughly the same origin time as ASN.1) and it was massively better in terms of tooling. Even at the time I tried to work with it, I had a hard time finding any decent C or python library that worked smoothly with ASN.1 (define schema, generate bindings, parse and serializes messages). Both of those are dirt-simple in XDR and pb.