I think it was probably just too much extra work to expect wide adoption of the standards. It could never be strictly enforced by the time it was laid out, and the lack of enforcement meant it was a classic chicken-and-egg problem where you couldn't expect people to spend a ton of effort setting something up when no one else was doing it.
It turns out though, that improvements in NLP mean there is still hope for a slightly different incarnation of the Semantic Web. There are a lot of companies out there now, like Diffbot [1] or Google [2], that are providing semantic searching of web data with structured results. It's not exactly the same as the concept of the Semantic Web, which would've required content publishers to tag their own data, but it does move towards the Semantic Web goal of making web data processable by machines.