Is there any reason why they can't just be installed inside air purifiers/ventilation?

Especially since you could probably get more mileage out of the same amount of light energy by forcing the air through a narrower passage, since only airborne particles are actually going to absorb any energy anyway, and air is mostly just empty air.

That solution is actually suggested in the article. However, that still leaves the cost issue mentioned by GP, and until that is solved, this will remain something for specialized applications (operating rooms) and maybe hypochondriac millionaires...