It's simple Nostalgia + Means (+ Scarcity Of Better Stuff)
I've spent my life going to vintage motorcycle swap meets with my dad and the trend is not limited only to vintage basses...
In the early 1990s:
The folks with the means to acquire the motorcycles from their youth were old enough that the bikes from their youth were Harleys or Indians. These bikes and their parts were very expensive, but available.
The folks who weren't quite "collector" age (and had no means) had grown up with British bikes, like Triumph, BSA, Norton, etc. The real collectors weren't into these bikes, so they were cheap.
The folks who were kids grew up with Japanese bikes. The British-enthusiasts of the 60s and 70s knew the Japanese bikes were outstanding but were bitter, and the real collectors thought they were junk.
In the early 2000s:
The folks who had been the collectors of the early 90s either passed away, already got everything they sought, or gave up looking. The old Harley/Indian stuff became mythical.
The folks with the means to acquire the motorcycles from their youth were old enough that the bikes from their youth were British. These bikes and their parts became very expensive, but available.
The folks who weren't quite "collector" age had grown up with Japanese bikes. The real collectors weren't into these bikes, so they were cheap.
In the early 2010s:
The folks who had been the collectors of the early 90s either passed away, already got everything they sought, or gave up looking. The old Harley/Indian stuff became even more mythical.
The folks who had been the collectors of the early 00s either passed away, already got everything they sought, or gave up looking. The old British stuff became mythical.
The folks with the means to acquire the motorcycles from their youth were old enough that the bikes from their youth were Japanese. These bikes and their parts became very expensive, but available.
It's funny. When I was a kid, people talked about 70s AMF-built Harleys the same way they talked about Japanese bikes (basically that they were lower than junk). Today, people are paying insane money for those 70s Harleys.
A similar trend can be seen with guitars. When I started playing in the late 90s, everyone was snatching up 50s/60s stuff, and had been for a decade, and as those have become mythical, we're on to the 70s gear. Try finding people in the late 90s who revered 70s gear. It's just like trying to find someone now who reveres 80s gear, but that stuff will have its time. And eventually people will be talking about 90s gear, and so on.
Nostalgia + Means (+ Scarcity Of Better Stuff)
Jacob