Reportedly, some members of the Donald's family forgot to register (the GOP NY primary was closed).
Both party primaries were closed-it’s been that way since 1976, and has been the subject of a pretty high profile lawsuit.
The problem was that they were ALREADY registered as Independents, and changing party to GOP had to be done way back before October. NEW registrations could happen just a month early but CHANGES had to be done more than six months ahead. Kind of crazy, but it does prevent Dims from messing up primaries with operation chaos antics.
That is dumb. They had to register a like six months before Trump decided to run.