Quoted from LesManley:
It is because the game is difficult and the theme is bad. Lots of people don't want to play a difficult game. Fish Tales, for instance, is easy and straightforward and a theme that most can relate to in fishing. If a casual pinball player could pick between Fish Tales and Shadow at the same price, they are going to pick Fish Tales. The more hardcore players are going to pick Shadow for the opposite reason. Problem is, there are a lot more casual players than hardcore players.
Two things to add though -- The Shadow doesn't have pops, a HUGE turn off to casual people who are interested in pinball who think they need to be on every machine, and the ramp diverter stuff is just confusing until you have done it for a while.
I think that NBAFB is a very new player friendly game for the most part, but every time they hit the scoop and then don't know what to do as the game is going, "LOOK UP!! SHOOT THE BALL UP THERE!!" it reminds me that even simple stuff can be really confusing for people that don't play regularly. Shadow with the extra buttons is even more confusing.
The fact it commands the prices that it does is honestly a testament to the gameplay. JD, another great player in my opinion with similar issues (no pops, but I'd argue a slightly better theme) is worth a lot less...