Because housing is inelastic and in short supply -- everybody needs it and if the price increases they will still pay for it.
If you increase the housing supply, the price falls. Increase it to a point where there are some vacancies and suddenly landlords don't have as much bargaining power to take your entire UBI. At some point a landlord is going to say "well better to fill it for $900/month rather than let it sit empty".
I think a huge part of UBI is making the economics of the essential things in life work out so normal people have the power when it comes to basic needs.