The reason is that all official servers have addon checking enabled. Heres how it works:
You load a mod when you launch, each mod is added to a list of "enabled addons" that are running during that session. when you join a server, your client 'pokes' the server with some packets of information, including your GUID, player name, profile information, and a list of your enabled addons. At this point, the server creates a join request with your information, and checks against certain variables, such as how many slots are open, if your GUID is banned, and if addon signing is enabled. If it is, the server then looks through the list of addons that it was sent from your client, and cross refrences with a list of "allowed addons" on the server. If ANY of them dont match, the server rejects your join request.
TL;DR: You cant run ANY addons for joining official servers.