I managed to rip a lot of stuff from old internet archive copies of the Wargamer today using a site downloader:
LINK to main page
LINK showing everything inside it
I managed to recover:
SPWAW: 496
SPWW2: 61
SPMBT: 124
SP2: 1
The reason for the distribution is I think rather simple:
Internet Archive (and others) chose what to save based on popularity -- the more people downloaded something, the more likely it was to be saved.
In the early 2000s; running DOS was becoming harder and harder -- DOSBOX wouldn't come out until January 2002; so SPWAW was more popular at the time; since it was a native Win9x app.
So all the SP1 and SP2 scenarios were not archived because at the time, nobody was playing them.