part of the deal bmw made with williams was that williams had to build a sportscar for bmw. the 98 cars were with drawn for something simple like wheel baring failure in one car. the rest pulled for safety reason like the clr-lm in 99. in 99 they came out and won due to a combination of: toyotas bad luck, a bit of their own, and superior gas milage. they did run the 00 alms, with one race win, but, they didnt put the same effort/cash into it.
i dont see why they bothered though. i remember reading online that there was half or less the mechanics working on the car than the year before. they didnt even bother to show up at the race of 1000 years.
then bmw shoved a shoved a race engine into the m3, gave up on lmps, and concentraited on f1.
why do you think toyota stoped racing their gt-one?