I have asked myself the exact same question before. Usually people cite complexity as the problem. That is really a bad habit, because the longer we leave the problem, the worse it will get. Semantic Web is a sophisticated solution to a complex problem. It doesn't get any easier. I think also that making comparisons of simplicity to RDBMS is naive. Most developers nowadays are familiar with ORM and work with abstracted persistence, some never aware of the persistence mechanism. The persistence frameworks for Semantic Web (ORDFM) are not typically as sophisticated or evolved. Having said that, a lot of organisations are moving away from RDBMS and investing in NoSQL solutions of which RDF and SPARQL are in my opinion the best candidates.
A great case study I always point out when people talk about Semantic Web being to complex is the Bart van Leeuwen story:
http://semtechbizsf2012.semanticweb.com/sessionPop.cfm?confid=65&proposalid=4590
If a genuine full time firefighter (who puts out real fires) can use SPARQL and RDF in place of databases and proprietary formats to solve a genuine problem (data accessibility in emergency service), then there is little excuse for the rest of us not to. My point is that its not the technology that is the barrier, it is something else.