The issue has been thrashed out many times.
Basically, some dopers feel that avatars would lower the tone – even if they have to be manually enabled per-user, and would be off by default.
It’s quite an irrational position IMO, but unfortunately it is held by several of the mods, so it is not likely to change any time soon.
More’s the pity because I do find that avatars make it easier to keep mental track of who said what.