In recent years new protocols for externalising data have become popular. Schemes like Protocol Buffers & Avro are in common use.
Stellar uses XDR, a scheme from the 1980s which is (as far as I can tell) not popular in recent years.
I don't question the decision, but I do think there might be some interesting and compelling reasons for choosing XDR over other formats.
Why was XDR chosen?