AbdulAlhazred
Legend
I think the main reason for the missing elements is that the RC is part of the Essentials line. So if it is to be the main rules resource for new players joining the game, they want to make sure everything in it is relevant to Essentials players. So rituals, vehicles, artifacts, certain weapon properties, familiar and companion rules - stuff that doesn't show up in Essentials - is absent.
Which... I don't think was necessary, as they could have found a way to include stuff while noting it is from other sources, and it would have been nice for it to be a truly comprehensive resource.
Still, I understand why they did it, and what is there is excellent and the entire idea of having this portable rulebook is very appealing.
But they didn't bill it that way. They billed it as a comprehensive rules reference for 4e. Honestly it seems more like what they're saying is "well, this old junk is back in the 4e attic somewhere, but it doesn't even rate being in the comprehensive reference." I get there are limits on page count and whatnot, but if you're NOT going to cover 4e and only cover Essentials and maybe haphazardly a few random elements that you happen to have room for from anything older than bill it for what it is, a reference to ESSENTIALS, not 4e.
It seems like the article linked previously here is sort of a retcon of the spin of what the RC is. Now after it's out they say "yeah, well, it isn't ACTUALLY a complete reference, it only includes the stuff we felt you really should be using going forward" (or whatever the logic is for what got in and what didn't). Really, toss in another 20 pages and cover rituals, BCs, Familiars, and 2-3 other fairly minor things that are all that's missing. It really isn't much. Leave all but the bare bones of the SC system to the DM's Kit, explaining how the rules work really isn't REFERENCE material anyway.