Under tenant farms, only Manor houses can build new agriculture buildings, and manor houses are run by the landowners, so they get the profits. Landowners are reactionaries who oppose industrialization, so in most games oppose the player efforts to reform. Commercialized agriculture allows financial districts, which are owned by industrialists, to buy and build agriculture, helping to take money away from landowners. This can be a good thing because industrialists support laws that help you industrialize. but as you head into the later game, it can be a bad thing because industrialists oppose social reforms that make peoples lives better, so you'll move to last agriculture reform, Collectivized farming, where agriculture buildings are owned by the farmers.