From my perspective, semantic web (if we are talking about the same things) failed not because of mobile, but for a variety of other reasons. When semantic web was taking off, the web was very heterogeneous, and web authors could not be expected to write proper tags. In addition, it received little support from mainstream search engines like Google, and for good reason; while the intention of semantic information was to help indexing and other agglutinative tasks, by being invisible to the user, a malicious web page might misrepresent the nature of their content.