Look at most current TPC courses to see courses with less bunkers. There are lots of reasons for it, including the maintenance cost as noted. I think the 90's will be remembered for many design excesses, bunkering quantity included. As we return to a more normal economy (i.e. crappy for discretionary income activities) cost will become even more important in golf. We will repeat the trend similar to the sixties and early seventies where renovations are done with an eye to the, shall we say, practical.....
The other reason is that, as we have discussed, bunkers are almost irrelevant to tournament set up, so why use them, except to carry on the golf tradition? Add in that they are penal to the everyday player, and help the best players, and its a feature that should be reduced from a philosophical perspective in modern design.
They sure do look good, though, if properly done! That's why I keep a few of them around.