Earlier quoted context omitted.
The OP answers that question pretty throughly. Because pacman is niche and distro-specific and largely distro-controlled, and has no ability to incentivize developers who need to get paid for their work. This leads to Linux desktop having far fewer applications available to them compared to windows and mac.
If payment is your main incentive to work on software you should stay out of the FOSS space. Go write your iPhone/Android app or sell your stuff on Steam. Another little insignificant app store won't convince Adobe or AutoDesk to port their stuff to Linux.
Also this seems to be a confusion about what FOSS is. FOSS "freedom" isn't about "free as in beer", it's about "free as in speech." The GPL even welcomes commercial software, provided they include the source. I'd be thrilled to see a category of "paid FOSS" in the store, and I'd buy quite a bit of it if prices are reasonable and the software is useful.