Why are frame pads no longer readily available? When mountain bikes first became popular people were a lot more likely to buy a full set of frame pads than a helmet. Obviously helmets had all of the aesthetic appeal of a lycra covered fan-heater back then, however, frame pads were what you had if you really went off road and were serious about your mountain biking. They were not seen as frivolous, just sensible, a bit like a helmet. In the UK nobody would walk out of a bike shop with anything from a brand new Kona Cindercone to a Raleigh Maverick unless it was bedecked with at least one 'Cosmic Trail' frame pad in some garish colour.

As well as protecting your nether regions from the bike during an accident they also protected the frame from damage and made it a lot easier to carry the bike up stairs or over a fence. They had their uses on car-racks too. The benefits were undeniable even if the florescent did have a tendency to fade.

Then, one day, when nobody was looking, they were suddenly removed from the bike shops and made no longer available for purchase. Were they that good that the powers that be in the bike trade had to kill them off, much like GM and the electric car?

Why have they not had a continued place in the market place? Given today's 'scratch 'n' snap' carbon frames and the fad for downhill-mountain-biking one would have thought they would be more popular than ever. As it is you only have a few pretend-messengers using them to hide the fact that their bike has got braze ons for the back brake and is not a true fixie.

Here is an advert for what was the UK's market leader - 'Cosmic Trail'. The all in one top tube and stem pad had the added advantage of smoothing out your steering making the bars less prone to being jolted by rocks etc.
From my perspective (normally a road bike user, and fairly good at falling of an MTB on rapid downhills) when I fall I always try to leave the bike, as pedals, cranks and sprockets hurt legs. 

So to me the most important things to protect are head, elbows, knees/shins and back. Frame pads aren't going to do anything for me, but protective clothing will.