Because a web-app is the only way you can monetize a desktop app like functionality in 2019.

Linux desktop toolkits and dev environment utterly sucks. You'll have to develop 3 different codebases for Win, Mac and Linux. Worse still, you'll have to reinvent app updates technology if you go with Qt or will be tied into a different updating technology and still have to deal with piracy problems.

Or you could go with web-apps:

-> Easy to develop code

-> Can't be pirated

-> In built model of regular payments

-> One codebase for all platforms

It's not even a close fight.