In addition to the obvious answer of "because our software is really good!", IMO an under-appreciated reason is that KDE truly is an anarchic and largely volunteer-run community. As long as there are passionate volunteers, there will be KDE; you don't have to worry about it just dying one day should some big corporation pull the plug for some reason. We've all become so accustomed these days to software being disposable, but KDE really does give you a measure of longevity and continuity that you're unlikely to get elsewhere, especially without paying a lot of money for it.

I think everyone has different preferences on how they use their machine. However I would invite to just try Plasma out. One thing where it stands out is in my opinion that you can tailor it to your wishes and use case.