I wish I remember where I had read something about Lego talking about how long it can take from concept to production. It might have been a Lego documentary or something. Anyways, if I remember correctly, they mentioned that I think on average in can take about a year to a year and a half to go from first concept to release. And I would assume longer in some other instances. You figure with a set as big as the Helicarrier, and Ghostbusters, it's entirely possible that before it was actually released to the public, it had already been in the pipeline for close to 2 years. So I think for the most part, it's just bad timing overall really, and not Lego purposely going out of their way to rip off people's submissions. Granted I'm sure they do some research into how popular something can be to make a new line/set what have you.
Before this gets too far off the rails, I'm with the consensus that most of the IDEAS submissions, while very nice, and would love to have them, are so insanely impractical for Lego to actually produce. Ranging from licenses that Lego already has, to licenses that lego will never get, and sets that are too big to ever even be considered. And that's the unfortunate double edge sword with regards to the IDEAS program. Sure they have gotten some new licenses, but the more popular sets, that I'm sure would sell really well, can't be done in an IDEAS form, but then the only items that can do well are original creations, and that can create a more niche selling market.