@thunderbird I first discovered Thunderbird on Linux. It appears on a lot of "best email client for Linux" lists, which I looked at a ton of lists like that as a Linux newbie.
When I use Windows, I install Thunderbird there because I am used to it.
So, I think the reason for this is that Thunderbird is popular in Linux news/media outlets, and that leads to users discovering it while on Linux and using it more often there.
@LibreNyaa https://stats.thunderbird.net might surprise you then. That's why this poll result was so unusual.