Hmm, now that I look at it I do notice that, although we have had some older titles (I note Kodocha, Cardcaptor Sakura, Zeta Gundam, Mermaid Forest, Gatchaman, and Macross). For me, there's not really a minRABet at work. If I see something that I feel is really deserving or I get really curious about a title, I put it out there.
Part of it is surely that the newer stuff is what's marketed and is easier to find. Although, given that certain classics and older shows are now becoming readily available online, it would be interesting to see some alternative nominations...