The word duct doesn't appear here because far-UV installation is most useful (compared with other wavebands) for whole-room application--agree that UV in ductwork has the potential to be very useful, but you'd use a longer wavelength if people weren't going to be directly exposed, because you can crank up the power on longer UVC wavelengths without producing tons of ozone. (Far-UV is specifically 200-230 nm, but UVC goes up to 280 nm.) I focus on far-UV because the excitement about far-UV specifically has to do with whether there's potential for it to be installed in such a way that it can stop in-room pathogen transmission before air entirely circulates or recirculates, which could be a big deal for pandemic prevention.
Why in-duct UVC isn't more widely employed: I don't know a ton about its current adoption level but retrofitting ductwork is a hassle, even just in the simple sense of needing to contract technicians to do it. Anyway, its efficacy isn't guaranteed; you need to know the power of the light you're getting, which many consumers are not equipped to assess. The efficacy is a combination of the light's power and the speed of airflow through the duct, UV lights are often sold at an insufficient power to effectively disinfect air, and consumers can't necessarily evaluate whether it was a good investment (common problem across air cleaners in general).
My guess is that we're still on the leading edge of ordinary consumers thinking of air quality as something that they can and would want to control for anything beyond comfort, particularly for pathogen (or allergen, or mold) removal. All air quality tech faces that problem (and my blog post was trying to address the problems far-UV faces beyond that). Anecdote I found interesting: Some colleagues attended a global indoor air quality conference earlier this year and reported that something like 80% of the sessions were about comfort in the indoor environment, while about 5% were about pathogen control.
(Austin is very kind--I am not close to being the world expert in far UV deployment; there are people who run/used to run companies trying to do that, and researchers who work with them very closely, who know more about far-UV deployment, and I'm largely consolidating information from them.)
Why there isn't more ground-up interest: it's expensive and people can't easily tell if it's worth the cost. Also anything where UV is touching you has to overcome people's safety concerns.
Good question on the large effects are easily measured thing--has to do with the distinction between: 1) in what environments you are cleaning the air, 2) how much you are cleaning the air there, 3) how much pathogen people inhale, and 4) how much pathogen is required to actually make people sick. It's not just a far-UV problem, it'd be a problem for any air cleaner, it's just that far-UV is especially expensive to install and especially faces negative "UV" associations.
Far-UV has a large effect on airborne pathogen concentration, and that large effect is in fact easy to measure in a chamber! But once you add it to a room where people are moving around and talking to each other, how much pathogen are they actually inhaling? Is the air in the room well-mixed? Is the far-UV reaching the infectious air before people inhale it? Even if they inhale air that has living pathogens, are they getting sick? If they get sick, did they get sick from that room or from a different environment that they were in previously? Study endpoints matter a lot.
Being able to understand intervention efficacy especially becomes a problem if a disease is largely spread via superspreaders/had high variance in infection sources. COVID, at least early in the pandemic, had very high variance, whereas eg seasonal flu doesn't usually. Therefore, if your study intervention is installed in public spaces, it's possible for it not to show much effect on seasonal flu but a large effect on a disease with early-COVID-like dynamics, which means you have to wait for that COVID-like disease to come along to see the effect--but that would be worth it; high-variance diseases are very concerning!
Another way of saying all this is that it's not the case that the effect will be super hard to measure given enough people and time, it's that the effect is hard to measure given that you need a lot of people in your study to account for the distinctions listed above, and/or you need a highly controlled environment, and that's just expensive.