Why are legendaries so impossible to catch or is it just a bug

Me and my friend recently started a cobblemon modpack server and we encountered articuno. We used about 50 ultra balls 64 great balls and i cant even count how many pokeballs and we couldnt catch it.

I put it to sleep and confused it and i still cpuldnt catch it. Is this a bug or is it just how legendaries are?

I should also mentioned that i increased the pokemonPerChunk from 1.0 to 5.0, idk if that makes any difference. And i also removed the level cap on the trainer cards from rctmod.