Zoning limits the competition for property to single households. I can compete and win against a lot of single households for land. I could not compete with 10 households looking to build apartments. It is illegal to build ten apartments on most lots though, so good for me I guess.
Landlords own land and make it available to others to use. They generally want to build more housing because they like money. They are a strong ally in the fight for more housing (which means cheaper housing). Single family homes are the enemy.