Why are there only NOR and NAND flashes? According to my lecture and wikipedia NOR and NAND flashes are the most common used. However there isn't really explained why people decided to use these logic gates instead of others. Can someone explain me, what are the factors deciding which gates to use and show how these gates match those properties best?
NAND and NOR are the only two binary gates that are functionally complete, which means that using just one gate type you can implement any circuit.

This greatly simplifies fabrication. As long as you can construct a single type of gate many times using the same technique, and can connect them arbitrarily, you can create any circuit.