I think a lot if it is that there are a certain number of basses rolling off the assembly line at Fender that (for whatever reason) sound far better than average. And within that better than average group there’s a certain smaller number of exceptionally good sounding instruments. And since they sound so good, they tend to be treated better and passed on more often than the average ones do. So it might be more a case of survival of the fittest than anything else. Exceptional instruments tend to survive multiple owners and retain their value. So what’s around from the 50s and 60s in the vintage market is the cream of the crop. Which might lead people to believe every 50s and 60s era Fender sounded and played great despite that not being true. Fender has made many exceptional basses throughout the years. There’s new ones being made as we speak. In every production run there will be a certain number of great basses. The trick is finding that special one in a herd of merely good instruments. The designation ”vintage instruments,” as most people think of it, isn't a badge of quality because of the age of the instruments. It’s more because those older instruments have been vetted by their various owners over the decades and the real gems have been culled from the herd. Exceptional instruments tend to survive the passing of years and multiple owners. Average instruments tend to fall by the wayside. A bass isn’t good simply because it’s old. However, there’s a strong likelihood it lived to be old because it is so good.