I have a reasonable guess as to why. I bet after a plate isn't used for two years, the state marks the plate number as unused and can re-issue the number to someone else.
So your number may be in use on another vehicle. Most likely it isn't. Being the government (and especially the MVD), they can't be bothered to simply look up your old number and see if it's been reissued (I doubt they even have a process of taking it off the available list). Easier to shaft the customer, because they have no market pressures that result in good customer service.
O2