That's because the minimum number of points for a master contract is larger than that for an add-on; a Master Contract is what defines the UY.
The only way to end up with a Master Contract that is < the minimum is picking it up a small Resale for that UY. It would create a separate master contract for you, but then I believe there are difficulties access points from separate different UY's under the same account to make a single reservation.
At least that's how the administration folks explained it to me.