Variety of reasons and periods. My Raleigh full size bikes all sport 40 hole rear and 32 hole front. The later ones are 36/36. I also have 48 hole tandem rear wheels.

As a general rule the less holes the lighter the wheel, back in the 1970's the materials and engineering had not developed to the point where you could anything under 28 holes. It has now. When I was racing circa 1974 I ran 32 spoke rear and 28 hole front, but I was a lightweight at the time, but even then could still collapse a wheel if not careful.

Aaron