As with any open source project, older hardware is better supported because it's cheaper and it's available for time long enough for open source drivers to be written and exploits to be discovered.
For example, running custom code on the iPod Nano 2nd Gen has only been made possible three years (if I'm not wrong) after the iPod started being sold. I think this is, mainly, because the firmware was encrypted (something that happens with most recent players, specially those from big brands like Apple).
For something actual that runs Rockbox I've been keeping my eyes on the Lyre project, but nothing really ready for massive production and "average-user" use has been developed yet. AFAIK, with Lyre we have a couple (?) open source boards that can run Rockbox, but you see, it's not something that comes in a nice box, with a nice headphones, a nice printed instructions manual, and all those things the 'average user' is looking for in a brand new DAP. Plus, like with any open source project, you really don't have any guarantees or guaranteed technical support, something the 'average user' seems to valuate a lot.
OOPS.. it seems two people replied first. Anyways, here goes my reply for whoever wants it

Is it true that most of the supported DAPs are old and are becoming difficult to find in stores (if findable at all)? Yes it is.
Is it true that no effort has been made to support more and newer hardware? I think I can say this is completely false.
Is it true that Rockbox is ready for use (stable) on these new DAPs? No.
Isn't it amazing how far we have got? With currently 45 targets for which a build is made every SVN change, support for over 20 sound codecs and the potential of every supported DAP maximized to much more than what they could do by default when you buy them in a store - I think this is amazing for an opensource project, and we shouldn't complain. Furthermore, with Rockbox you won't find many bugs, and those you find, will most likely have already a solution in progress (at least, this is true for my DAP).
The Rockbox Community has even created a page to help people willing to buy Rockbox-able players to choose:
http://www.rockbox.org/wiki/BuyersGuideAs shame, it seems to be really outdated. For example, it doesn't include a line for my iPod Nano 2G. But, check it if you haven't done so and see if it helps you.
It seems I'm finding my way to ask for write permission on the wiki, because I've already seen some things a person with spare time (like me) could easily update.