Why are major league baseball fields not standard in size?

[Butch7999]

Not disputing nor disagreeing with the overarching sentiment of anything said above, but just for the record,
NHL rinks weren't completely standardized until the 1990s -- Boston, Buffalo, and Pittsburgh among those
long deviating from the otherwise uniform 200x85'.
Wasn't a 1930s NFL Championship -- maybe the infamous 1940 game? -- played indoors on a drastically
shortened field?

Edited to add: To actually address the original question, we'd guess it largely had to do with the situation
that the 19th Century rules provided exacting specifications for dimensions of the infield but said little
about the outfield nor about the amount of space allotted for foul territory. Given the availability or
unavailability of existing venues and the expense of building a new stadium. and estimating the volume
of potential spectators, clubs went with whatever dimensions they deemed the best fit for their team
and for their budget.