Because you use to many gates in your RAM for example you use 16 OR this is 15 * 3 gates = 45 gates , if you will use 3 state buffer you don`t need use OR
Output can only have one connection , and you use OR gates , but if you use 3 state buffer you can make 3 type of signals [1 0 no connection] no connection allow you connect multiple signals to output BUT only one can be on !!!
if you have 8 cell memory you only need 8 3 state buffer (one per cell) address pin connect to enable data to IN and finally you will get a some extra steps in simulation =D
if You want I can send you my realisation of RAM