I think that the 'collectibility' of vintage instruments is more down to sentimentality and myth than anything else. The main brand which does well in the vintage market is Fender, which is probably because they are the biggest company in the market, they're the most copied company in the market, they've been around for longest, and people remember wanting Fenders when they were kids because their hero played one. There is also the perception that Fenders of various periods are better (pre-CBS, etc), but because all of the pre-CBS models have been snatched up and now command astronomical prices, people moved onto the later basses which are now all more expensive, and now even the 80s stuff is starting to rise in price, which is funny because as far as I was aware, everyone seems to think that the 80s stuff is crap. Because of this, the market is huge, demand is huge, and sellers can ramp up the prices because people will pay them.
Warwick doesn't have any of that stuff. They've only been around since the 80s, no one makes copies of their instruments, and the only people whose childhood heroes played them are people in their mid to late 20s because of the Nu Metal wave of the early 2000s (I'm one of them). The number of buyers for Warwicks is tiny compared to Fender, so if people try to sell Warwicks for high prices they just don't sell, because the demand and the mythology just isn't there.