Why hasn’t NAC made 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 only recently found the proper type of plastic substrate of a thickness to make (up to) 90 minute cassettes. Since NAC began manufacturing magnetic tape until now only a substrate to make 64 minute or shorter tape had been available. Any longer cassettes available anywhere (that we know of) have been made with older stock, not newly manufactured tape.