Still Telekom has a big market share there.

They're entrenched. They have Telekom-branded stores (resellers) in every other town. They operate the network and hence have the best ability for trouble shooting. Non-technical users don't know what peering means, they just pay for Internet like they pay for water or power.

So for the "commoners" it seems a solid choice, while we, the Lords & Ladies of tech, are cursing in our basement home labs ;-)

Also, and that's why I'm stuck with them, for some reason they're the only one who offer combined DSL with 5G "boost". Our line is limited to ~45 MBit/s, and we get another 100 MBit/s over 5G. Doing this yourself with multiple links is of course an option, but costs a magnitude more than the 5€ extra I'm paying now; and the day only has so many hours to take care of such private deployments.