I think NEC was against the image of a "family" console. They wanted their console to mainly be a one player experience. Plus, if NEC added extra controller ports, then how could they sell off all of those taps they made?
Better question is how could they sell off their GAMES with the multiplayer feature instead? It bothered me how the Turbo grafx lacked a feature that was common sense in the console industry. Hell, even the CD-I had a second controller port, albeit in the back. You would have to pay for the second controller on most other console but with the Turbo grafx, a 2nd controller AND Turbotap was necessary.
A few weeks ago, when a buddy from work came over to play some games, he opted for a shooter game and I suggested Soldier Blade. However, he wanted to play together but I didn't have the Turbo Tap. Tried to persuade him to try it by himself but he insisted it was not right for me to be "left out" while he was playing.
If your buddy came by to play a 2 player Genesis or SNES game and you didn't have a second controller you'd be in the same boat. Not only did the Genesis normally
only 
ship with only one control pad, but all the 'chumps' who shelled out for a second controller wound up having to buy
two more 6-button controllers after street fighting game became all the rage. Even as a jobless kid a Turbo Tap seemed very affordable and at least it supports as many as 5 players plus there are some great 5 player games even on the North American side.
We'll never know how much of an impact having a single port and pad out of the box made on the Turbo market, since pretty much everything else was handled to poorly. Maybe instead of having 5 - 7 pack-in games with the TurboDuo, they could've included only one or two and instead thrown in a second pad and a Duo Tap. But it was obviously not a deal breaker for the market it was designed for, since the PC Engine was a huge success.
What is much more common sense today than a second control port was back then, is having free online multi-player service out of the box. But both the Wii and 360 don't have this yet both are major successes while the PS3
does and is trailing behind. Hell companies today are selling games that require huge expensive dedicated gimmicky controllers and people still buy them all, even when a complete Rock Band/Guitar Hero set costs nearly as much as the console itself. A Turbo Tap seems like a pretty reasonable purchase compared to Nunchuks, Webcams, Congas, Maracas and Fit boards.
