I can easily explain this.

Two buildings are for sale. They are identical in rental income, quality, etc.. and cost the same.

The only difference between the two buildings is that one has enormous restrictions on how you own and operate the building. You as a landlord will have a cap on your potential upside in rent, but NO cap in the downside.

Which building will you buy, all other facts equal? Unless you are insane, it will be the non rent-controlled building. As such, the price of the rent-controlled building will have to be lowered to compete for a sale. Hence lower prices.