To sell multitaps?
I sincerely think that it was one of the best choices made by NEC and Hudson : multiplayer games came very early in the lifespan of the PCE and games such as Fire Pro Wrestling -which is quite a piece of shit but was very popular in Japan-, Bomberman and Motoroader were already 4 or 5 player games. So anyway, you couldn't do without it.
On the contrary, the SNES and the Genesis both had 2 controller ports, and still you needed a multitap to play Bomberman on these systems.
Hudson and NEC clearly designed the PCE as a multiplayer machine really.
At least that's my opinion.