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At many hotels only certain types of rooms are available for rewards. So if that type of room is sold out, then reward availability can go away.

But in any case, any hotel can suddenly get a tour group booking which jumps up their number of rooms used by a large number in one fell swoop. Or, if you haven't been counting inventory, they could have only had one reward room left, and someone booked that one room (whether as a reward or with money).

It generally takes a cancellation to make a hotel reward "come back" after not being available. Of course, hotel cancellations can happen all the time, so that's not impossible, but OTOH it's not something you can count on either.

What you apparently didn't know that could have helped out: Best Western, unlike other programs, does not require you to have the points in your account to make the reward booking, only to have the points in your account soon before your stay (I've seen hotels pull the points a few days before my arrival, but never months before). So you should have made the reservation while you were waiting to get the points, when there was availability.