Property is trying to protect its suite inventory. This has become even easier for the properties this year because MR, in the wisdom of those who run the program, decided to specifically exclude suites from the upgrade program. This is not to say, however, that certain properties won't upgrade a plat depending on availabiltity and other variables including front desk staff. The new rule has, thus, made suite upgrades entirely at the option of the property, and some properties graciously provide suites to plats. It also wouldn't surprise me if there is some unofficial MR policy encouraging properties to provide suite upgrades to plats because a number of posters, including myself, have noted that in our real-world experiences, we continue to receive suite upgrades, on occasion. (For me, no less often or more often than before the rule change.)

As to the particular property you stay at, I would have a polite, non-confrontational conversation with the manager, pointing out the volume of your business at that property, and asking him/her to waive their normal rule. Good luck; you deserve that available suite.