Because he can't alone arbitrarily raise the rent and lose you to another landlord, but if across the board people suddenly had $XXX more money per month it would be stupid not to increase the current rents for that amount (assuming they aren't a slumlord). Most people don't want to move away from large population centers (where most rent payers live).

Also when the experiment begins there are exactly zero guarantees that it will last more than one election cycle, so would you by immediately quitting your job and moving to a cheaper city to enjoy couple years of UBI or would just pay the extra cost in rent from the "free money" you get from the government and wait out if the experiment continues or not?

I can easily see that increase to be permanent if the program is seen as a success and then people are left with the same decision of moving out of the cities where they have their lives established in favor of not having to work for a living.

I grant you that I have a very pessimistic view of the future and it sure would be wonderful if I could just dick around playing WoW all day without having to worry about bills, but the little devil on my shoulder is constantly whispering scenarios where all attempts at making world better through charity just end up with us in a worst place than where we started.