Microsoft's head of marketing at the time posted about this on internal Yammer. He said there were two main reasons: first, "Windows 10" just did better in focus groups than Windows 9 or other alternatives; second, since the plan at the time was for all future changes to be delivered as updates rather than a newly branded Windows release†, it felt better to end on a round number.

† btw, what became Windows 11 started development as just another Windows 10 feature update, and what gets branded as a "new release" vs an "update" is mostly a marketing decision.