The A series was pretty popular several years ago. I think that the main two reasons they're not as much is that prices have gone up and there's a lot more competition now. Denon's, Ultrasone, and now the JVC HA-RX700/900 for instance. I directly compared the A900 and D2000, and while I still liked the A900 a lot, the D2000 did have a bit nicer bass and the earpads are much better. The JVC's have been compared to the A series, and well rated in general, and are only $30-70 so that's definitely had an impact. I got my first A900 for about $115 used, and the new one I got a couple of years back was only like $150. I think if the A700 was the same price as the AD700 it'd be a lot more popular. I think the M50 is sorta taking the place of the A series right now, which is why we haven't seen a real replacement for them.
There used to be a fair amount of A900 fanboys, which made the limited edition versions worthwhile for Audio-Technica to make. I was in this group (I started out as a Sennheiser fan, but loved the A900s when I tried them), and even bought the A900Ti brand new. I did like them, but they were fairly overpriced. Not long after that I nabbed a pair of W5000s and have been a fanboy of them since. I've actually only heard the A900s, but I should have a pair of A700s to listen to in a few days.
I have felt that the A series has been underappreciated though, and some of that might be that there's no high end modding like the Denon's have with Lawton and the DT770s have with Headphile. If I had the capability of making some nice cups, I probably would consider doing something similar for Audio-Technica headphones. Larry at Headphile actually did mod a pair of A900s, although I was surprised he replaced the whole cup/housing instead of just the end "cap", and I didn't find them especially aesthetically appealing because of it.