There are some good points made here. I think the sheer amount of line up changes perhaps put people off, as well as those who weren't willing or able to accept anyone other than Ozzy or Dio, and just wanted the style of the early records. If they had been released as Iommi solo albums, maybe they would have been shown far more respect. Certainly I would rate them as highly as most of Ozzy or Ronnie's solo work.
One thing though, while the faithful, and certain younger metalheads curious about this legendary name, went out and bought the albums, and went to the gigs, I remember as a kid reading reviews that seemed to suggest that all Sabbath was old hat in the era of speed metal and glam. I recall when I started buying metal mags like Kerrang in 88/89, and reading reviews where new bands (e.g. Acid Reign) who even dared to cover Sabbath were slammed, so people in the press, and the casual observer were never going to give them a fair shot.
As far as Cross Purposes and Forbidden go, the backlash from Dio's departure, the increasing interest in the Ozzy era (and the media bias against melodic/80's metal) from fans of grunge (and doom/stoner rock to a certain extent) and the fact that Forbidden was painted as some trend hopping rap metal abortion all played their part. People simply don't give those albums a listen, which is a real pity. I know people who would love them if they would only give them a listen, but they are so entrenched in this image the press has built up of post Ozzy and/or Dio Sabs being "naff 80's rawk", they won't even take a chance. A lot of great music from the 80's and early 90's is unfairly dismissed, not just Black Sabbath.
Frankly, my favourite albums remain the Ozzy and Dio era ones, but I love the Martin era stuff too (as well as Born Again and even Seventh Star) whatever people think of them being released under the Black Sabbath name, there is no doubting the fact they were, and remain, very good heavy metal albums, with quality songs, some of Iommi's best playing, and stellar musicianship.