Because KDE as a major and old open-source community doesn't like to use proprietary hosting services when they have their own infrastructure (and probably had their own infrastructure before Github even existed?)

You mean why they are using git as intended instead of introducing a single point of failure? ;-)

Many in the free software movement shun commercial software. To make things worse, GitHub recently got aquired by one of free softwares old enemies.

Personally I've used and even recommended GitHub before Gitlab became available and I don't shun them or even Microsoft, but I also see where they might be coming from.

Don't use "commercial" when you mean "proprietary". The free/libre software community isn't opposed to commerce, only to restrictions on liberty.

> You mean why they are using git as intended instead of introducing a single point of failure?

If you could introduce a single point of failure merely by using git with a proprietary remote service, then you are using git wrong.

There is currently a test for the adoption of a KDE instance of Gitlab CE (like Debian and Gnome) : and that should really help onboarding new contributors

https://invent.kde.org/kde/kdenlive

Sadly they aren't planning to mitigate their issue tracking into gitlab which will severely hamper that on boarding. That being said a lot of KDE devs prefer seperate user facing issues to their development space because of how many bugs are submitted daily.

Do you have a problem with the KDE workflow, or do you just want it to use tools you're more familier with?

Not all open source projects are on Github. In fact Github doesn't even host the majority of it.

Why would KDE host their git repos on Microsoft's propriety on-top-of-git service?

It's on their own git server: https://cgit.kde.org/kdenlive.git/