Hmm...I'm sure the scarcity of this bikes has
nothing to do with a web forum thread about three speeds that's gotten to about 726 pages in the course of eight years, a thread where people regularly praise the pre-1960 bikes as The Bee's Knees and disparage the post-1960 bikes as machines likely to disintegrate going down a hill. Nope,
nothing to do with that!
Anyways, to be more serious, the rarity probably revolves around two factors:
scarcity and desirability. While pre-1960's Raleighs et al were not rare on this side of the pond back in the day, they weren't
common, either. Adult biking wasn't really a thing from 1945 to 1960 in the US and probably Canada. It wasn't until the 60's where adult biking became a "thing" again, and the numbers of adult bikes increased here. It's not just British bikes: you can definitely find a fair share of 60's era Schwinn Varsities, Collegiates, and Continentals if you look, but you'd be hard pressed to casually find the 50's adult Schwinn frames.
Add to that the desirability of these pre-1960 bikes, as they have been talked up as being better than what came after, so people seek out those bikes instead of the easier to find newer versions. And they're probably more likely to hold on to these bikes vs the newer ones.
It's not just pre-1960 British roadsters. Look at 1980's touring bikes. They were common back in the day, but not in the numbers of sport-touring bikes. They were forgotten about for awhile, then people realized how nice these bikes are, talked them up, and the prices went up accordingly. On Portland's Craigslist it's rare to find one of these bikes for less than $400, regardless of condition. Of course, you can find a 1980's mountain bike that works just as well as an 80's tourer for the most part for far less. And the old MTBs use 26 inch wheels, so you can get wider tires too. But people aren't hyping up the 80's MTB to the same level of the 80's touring bike...yet.