Why? Because most stuff out there is not yet ready and will not be in the next couple of months.
What? The possibility to run X/sdl1.2/... stuff on Jolla.

Jolla might be afraid that giving oldtimers appspace on their platform will be hindering newapp development/new system embracement. Lets be honest, even if everyone embraced qt, this still leaves a ton(millions of tons) legacy stuff out there that will never be doable on wayland/qt5 without compatibility layer. Currently, Jolla the "true linux smartphone" works with 95% of bionic libraried software and refuses to work with 99% of opensource proper linux software, ironic? XWayland (X compatibility layer?) could change that radically and make the linux phone run linux software. cxl000 did some 'initial' mer-based x86 work on that:

https://build.merproject.org/project...%3Acxl000%3Amc
https://build.merproject.org/project...%3Acxl000%3Amc

More to come(I hope)

(and to make Jolla aware of the need vote up here: https://together.jolla.com/question/...uest-xwayland/)