Or cypress trees. Or pitchforks ... oh, wait.
I've never seen any indication that you can't base Cuusoo projects on old themes, and a two-second search turns up evidence that there are such submissions
. I'd love to see, for instance, one of Pete Reid's reworking of Classic Space designs as a set. I think, as Blue implied in her first answer above, it's just that the only ones voting for such designs are AFOLs with a real memory of those lines, and we just aren't a big enough voting bloc.
BTW, Blue, I disagree with your assessment on the new horses. The old horse was introduced in ~86 I think from a quick perusal of Bricklink. The basic minifig body was introduced in ~77 or ~78. So if the horse is aged out due to IP rules and therefor needs to be replaced, the fig is much more so. I think the introduction of the new horse is simply because they thought it added value to the sets to have a less static horse, and also a less blocky shape to the head. This is totally based on my own speculation, so I would be happy to admit if there is any evidence to the contrary.