You make a good point Brendan, sliders are indeed a very purpose built bike that's restricted by not being able to be used for anything other than what it was designed for. Unless you know somebody with a slider I imagine it'd be hard to get a ride on one to get the feel of what they're like, it's hard to imagine taking one down to the local paddock for a fang. They're distinctly different from just about every other form of motorcycling both in riding style and the construction of the bike itself which brings me to another reason for the decline in the slider......brand identification.
In this brand name obsessed world many of us become aligned with one particular brand of car, bike or even beer and we often become quite tribal about it. For example, it's so easy for young Tommy to become a Honda guy when he sees Honda bikes on the road, on TV motocross, freestyle or supercross and in advertising all over the media. The bikes are easy to tell apart from their Yamaha/Suzuki/Kawasaki/KTM opposition and it's a breeze to recognise the individual models, even from a distance. Best of all though is that they're cheap enough for Dad to buy young Tommy's first bike, a CR85 or 100, and simple for him to learn to ride at the local ride area. On the other end of the scale the speedway or dirt track slider is a pretty anonymous looking bike made by small companies like Jawa, Godden and GM, are never advertised in the mass media, making them pretty hard to earn brand identity brownie points when they all look pretty much the same. They're not products that can be easily sold to the public via mass media advertising. There's no multinational brand identification and loyalty factor and most importantly, they're extremely difficult to learn to ride without tutoring. The slider has unfortunately become identified as being a hang over from another era, like smoking rolly's, women wearing hats and contested rugby league scrums.