Why did they make 55mm lenses?
In the old days, for the 35mm film format, high quality 55mm (and 58mm) lenses were easier to design than other focal lengths such as 50mm. If you wanted the best f/1.2 or f/1.4 lens, then 55mm or 58mm was your choice. That is why most of the classic legendary high speed lenses are one of these focal lengths. Unfortunately for the lens makers, 35mm photographers didn't especially like the 55mm field of view which is why normal 35mm lenses migrated to 50mm. For 35mm full-frame, the more you deviate from 58mm, the more complex the lens design becomes in order to maintain high performance.
In today's world of computer designed lenses with special glass formulations and coatings, it is easier to design 50mm lenses with good performance than it was 40 or 50 years ago. Some of the old design restrictions are not as relevant anymore.
Here is a link to a discussion on photo.net that explains in more detail:
It's not how many pixels you have... but how you use them.