Was it an economical issue being that they had already been mass produced, and the resource cost to produce something else would have been a waste?
Was it a technological issue of not having a better baseline infantry weapon already developed, and therefore just stay with the tried and true?
You will likely find this old AMA on the small arms of World War II to be of interest, as it does touch on this topic a bit.
u/GeorgyZhukov answered a similar question about a year ago. Hope this helps!