At the risk of sounding like a landlord bootlicker....everyone thinks they're good pet owners but many people are awful pet owners and allow the dogs to do whatever they want and that can cause a ton of damage to a place, nevermind contstantly fielding calls from other tenants for barking or loud clacking or whatever because the other tenants get annoyed and it's simply not worth it. As you said, availability is super low so you also have your pick of tenants that don't have pets.