Many people here are answering the question of why RDF sucks, but that was not the question asked. The question is why this suckage has still not managed to bury the technology.
There is a very frequent problem people suffer from, which is mistaking goals for results. When you start looking for it, you'll see it a lot. A new open source NoSQL database will pop up, post a long list of goals ("Fastest performance, maintain some integrity, transactions, trivial sharding, consistent available and partition tolerant, and able to run on a TI-83 at web scale!"), put up a benchmark that shows that if you have no features and have no code written to ensure you don't fall down under real load you can put up way bigger numbers than the products with features, and suddenly you have some set of very excited people. Why are they excited? It's not the code; the code is worthless, the only thing it can do is run that benchmark. It's the promises.
You can see it in graphical programming. Graphical programming has a few modest successes, but the promises are about changing how everybody programs and how even Granny will be able to program. The fact that it has never happened despite immense effort for tons of smart people doesn't stop a certain segment of people from still being True Believers.
And, today, we talk about RDF. It promises to organize the web, it promises glorious wonderful search engines, it promises the world. It can't deliver, because merely sticking URIs on some graph nodes is only the beginning of the solution, not even remotely the end, you still have issues of agreement and accuracy and all kinds of other things. But the promise is so beguiling that some people just can't give it up, if we just try harder it'll happen, it's just that nobody has done it right yet, I'm smart enough to see what the previous hundreds of smart people haven't and I'll get it right, oh, it'll be glorious when everybody gets their heads out of their ass and listen to me and just start doing it right.
But RDF can't get us there. It's so general it's nothing at all.
There are all kinds of places where people become excessively bedazzled by promises and never notice the concrete reality before them. Another interesting example is Object Orientation. This has proved useful, IMHO, if not the be-all end-all of development methodologies, but it is interesting to contrast the promises made by OO back in, say, the late 1980s, with OO reality today. The promises were about how objects can represent things in the real world and you can model the real world with them. This turned out to be bunk. The real world has some place in OO but only carefully layered and wrapped and mixed in with a lot of other not-real-world things, iterators and factoryfactories and facades and data structs and ORMs and so on. The old promises were interesting and wrong, but also so beguiling that even today you will still hear this nonsense spouted about how this is the purpose of OO, even though it is now well understood that writing your programs with an excessively-strong tie to physical reality is asking for problems. Even as the reality is actually useful the old beguiling promises are still around screwing young developers up to this day.
(It is a tricky balance maintaining the proper level of skepticism because conditions change and sometimes wild promises become practical, and sometimes someone really does manage to pull off one of these things. The latest example would be the commercial success of the iPad, because for a long time smart money was on there being no market for tablets after numerous and repeated failures in creating the market. But in general, "show me the code" or appropriate manifestation is still the best way to avoid being trapped in one of these marketing traps, you will miss out on a few hits but pass on dozens of losers.)
(Also, I am aware there are still some True Believers using RDF. My point here is not disproved by a couple people using it, even using it in a big way. My point will only be disproved if someone brings about RDF Utopia, the actual promises. Of course you can bash RDF into submission, but that doesn't prove it was the best solution for your problem.)