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.