It's to prevent people from evading IP bans. IP bans are effective at making ban evasion more difficult.
They'll go through the trouble of getting fresh bridges like we did.
Using bridges doesn't help you evade the proxy ban. Bridges are only used as entry nodes. All exit nodes are publicly known. It would be technically possible to download the list of every exit node and completely
block Tor, though I have not bothered to do this yet.
Already many major proxies are banned. Obviously they all can't all be banned, but eventually it will be really hard to register using a proxy.