Well, maybe you're new to the line so here's a little history of SSC Star Wars. Sideshow's never made armored figures before and have been performing research and development for sometime. As has been revealed with the Clone Wars Obi-Wan, Sideshow's armored figures also feature a new body made to be smaller to keep the armor size true to scale as it's made of thicker material that would look too big on a standard body.
So, only very recently, Sideshow has begun it's armored figures, so it's not that Vader was passed over for less relevant characters, but they were not ready for him.
Add to that, there are features with Sideshow's PF Vader that many, including myself, see as imperfections, particularly when other pieces have been made without those issues, so there is a pressure with Vader to not only make him but to make him just right. Vader is pretty much the most important figure of the line in collector's minds and I'm sure Sideshow's, so his creation will be a labor of great love, attention and craftsmanship.