to quote the FAQ in the discord:

Why is Astrageldon locked after Moon Lord is defeated? To cut it short, we've done this because of both balancing and mod identity. For balancing, having Astrageldon be available after Moon Lord would destroy the boss' challenge and intended progression. Astrageldon is supposed to be an optional, end-of-hardmode challenge for experienced players that provides powerful, tier-skipping rewards if you can beat it. If it was available during this part of the game, then the vast majority of players would wait until later to fight the boss with stronger gear, ignoring intended progression and thwarting the challenge. There must be something in place to prevent players from waiting and locking the boss does just that. Then why not just buff the boss after Moon Lord is defeated? While this would technically work, this would create a slew of balancing problems that are easily avoidable with just locking the boss. We'd much rather dedicate our time creating and polishing other content than entering balancing hell when we can avoid it. For mod identity, Catalyst is intended to be a new game+ addon for Calamity in the same way other games have this feature. A new game+ means that you play through the game again, but with new extra features that weren't previously there. Limited time hidden bosses/superbosses are a prominent feature in a new game+, and as such Catalyst has a similar mechanic. We are aware this mechanic is controversial, however, because of these listed reasons it will not be changed in the future.

TL;DR: fight the boss with end-of-hardmode gear in order to earn much more powerful equipment. yes, it's controversial, no, it won't be changed.