Fixing statistical bias in a census adjustment is hard. In the 1990 procedure, there were 1,392 post-strata in all. Sample was about 380,000 people in 169,000 households in 5,290 block groups. Sample is about 5 times larger for 2000, but taken in much less time. Increased number of post-strata outweighs any improvement the larger sample might afford.
People are like fish.
All people in a post-stratum are equally hard to catch. No other effect of geography, etc . Undercount in a block is determined by the fractions of each post-stratum contained in the block. None of these assumptions is true. The failures add bias, enough to make the adjustments worse than doing nothing.