ET Pro has better hitboxes & netcode , but it has other limitations that stopped OZ making the move to it in the past.
I think Phil , Bishy , Blusky & Triggs have nailed this one on the head & I have to agree with Joke here too.
Pro doesn't really / if at all , support third party mods , & our mod was coded for pub as joke said.
Without admins / members no one to keep server running smoothly.
ET 2.60b patching issues as Bishy stated earlier.
Omnibot , yes everyone hates bots , but no-one will join an empty server & sit there waiting for a second player , so they'll leave.
Nearly all servers that died had limited (terrible or nerfed) or no bots to populate server , even noobs want something to shoot at.
I know all the pro's wanted the pro hitboxes , but there were too many unsolved issues to migrate to pro.