It's possible some protocol issues prohibit the use of other stream cipher modes like AES-GCM. I can't think of any but maybe:

* Dropped packets/out of order packets. You should be able to include the IV/counter with the packet but maybe the protocol prohibits that.

* Avoiding reusing counters between participants - seems like you could just use the participant ID as part of the counter and avoid this

* Concerns about partial packet loss - shouldn't happen with UDP and GCM would handle this just as well

I'm trying but I can't think of a good reason to do it this way.

Maybe, it makes the "Ministry of State Security of the People's Republic of China" job easier? It would allow them to easily access lots of data.