Why isn’t NAC making 90 minute or 110 minute tapes?

The overall thickness of the tape itself dictates how much of it can fit inside a cassette shell. The plastic film substrate + the binder + the magnetic oxide surface = the tape thickness.

National Audio Company has searched the world, but to date, has only been able to find sources for the proper type of plastic substrate of a thickness to make (up to) 64 minute cassettes. Any longer cassettes currently available have been made with older stock, not newly manufactured tape.