I'm less familiar with Halo 2 Anniversary's map rotations than I am with some of the other games, but assuming Stonetown is still included in 4v4 objective modes, the map was probably considered to be too large for 4v4 slayer, but still small enough for other modes with objectives that draw players to more specific points on the map.
Slayer is much less focused when it comes to map traffic, and tends have more instances of players camping to get kills. Due to Stonetown's combination of verticality and open spaces with less cover, in addition to the map's larger size (compared to most 4v4 maps), there are more points on the map where camping is effective.
The map is more suited for asymmetrical objective game types in particular. In fact, Bungie originally designed it specifically for One-Bomb Assault (obviously it was called Zanzibar back then.)