Microsoft, basically, had no clue: they relied on local partners to request L8N-related features. Then, they tried to figure out if those requests made any sense. For some markets, like Japan, there were existing industry standards that made this easy-ish, but for most other markets, it was pretty much guesswork, and, based on expected dollar-value, they would send out consultants to try to figure things out.
So, why was IBM a valuable/likely data source? Simple: they had been selling typewriters into most markets since, like, forever. So, they had to manufacture Selectric "typing elements" (the wheels and/or balls that produced the actual characters) to match the local market needs.
CP852 is most likely a more-or-less literal mapping of the Selectric typing elements that were most popular in the "Eastern European" markets.