The Crypts tend to spawn in the "thicker" swamp biomes only. What I mean by that is that if the swamp is just a small one, hugging the edge of some other biome, it tends not to have Crypts. I think the Crypts only exist in the "deeper in" parts of the swamp, kind of like how Black Forest has only Fir trees at the fringes and doesn't have Pines until deeper in.