I had 2 similar experiences recently with hotels in Niagara Falls and in BKK. The Marriott was unavailable for standard rewards and the Hilton equivalent was available.

Although I do know about Marriott's unavailabilty of a sort of "Platinum Guaranteed Availability" for reward reservations (other than stay anytime which is available to other tiers of membership as well), I can tell you I was taken aback. It got me thinking: what exactly are Marriott MR's competitive advantages over HH (its main competition)? Not complaining, I know what Marriott offers and what it does not, but I wanted to tell the OP his/her puzzlement is understood.