Krita has far more Windows users than Linux users, so does GCompris, and very probably Kdenlive too, to mention 3 of the bigger (in terms of userbase) KDE products.
And all of them combined have fewer users than Firefox and Thunderbird which both nag people to donate.
Many more KDE app projects are currently compiling their products for several OSes (Linux, Windows, macOS, Android and iOS)
With exceptions like Krita, Windows support is second tier for most (things like weird icon sizes and window positions) and even for applications that support Windows well, there is little interest in promoting the Windows version, even though there’s a decent chance this would improve the developer pool. I’m fairly convinced that had Windows support been a bigger priority, the KOffice/Calligra would have had a chance to survive against OpenOffice/LibreOffice, especially considering that it could have worked with Kontact the way MS Office has Outlook. Kontact even had reasonably good Windows support before the move to the whole Akonadi thing.