Initially players WERE able to get pretty good xp but that led to people spamming Portal with just afk farm lobbies that clogged the entire thing with basically junk servers.
Can't afk farm if the xp isn't really worth it, so they pulled the trigger there and here we are. It's a fickle balance cause if it's TOO attractive then you end up with just farm lobbies and Portal becomes kind of a slop fest instead of a cool custom games and server browser. Make it not worth at all and you stifle mode creativity and shaft players who dont wanna play pure pvp but also wanna level up and progress.