58's used to cost marginally more (might still)
whether it was just the packing in the grill, the grill it self, I don't know for sure but 58's always seemed to have a more pronounced proximaty effect. This made them less useful (to me) Proximaty effect is another tool in the arsenal, I just found it easier to control (when I had the choice) with the 57 then 58 (subjective, but just never experimented much with 58's)
when I started carrying a bunch of mic's around there were not as many readily available affordable options for (I think my first mic case was leather attache case a girl friend gave when at some point she thought I might wear a tie every day) for protecting them in transit. at the entry, novice level one size (mic) seemed to make sense
So?
SM57 was slightly more flexible, in a pinch you could use it on almost anything (& everything) live I got to recording from playing then songwriting and was familiar with 57, far less so with 58 (58's were for weeny vocalist, front man posers, etc.) you could drive nails, defend yourself in a bar conflict sing in the rain
reasons why SM57 became swiss army knife of mics for so many people are probably as varied as the individuals . . . I was never a huge fan of the SM57, migrated fairly quickly to 421 MD (which I prefer on almost any instrument (compared to 57), but they cost 4 times as much and I have actually had 421's die on me (have a unidyne with 57 capsule that is closing in on 40 yr (bought it in 71 or 72) and at least one SM57 getting close to 30 yr. and the 57's that would have been as old as the unidyne 544 didn't quit working, at some point I just no longer sheltered them . . . for all I know they are still suiting up and working every night