Just picking nits here, but the ATSC standard does include 24fps as an option. There are a total of 12 different NTSC HD formats allowed, including both 24 and 23.976. Check the Wiki.
In practice, all broadcasters in the US have chosen to pick just one of those 12 formats and to stick with it at all times. In addition, I believe they are all using one of only two HD formats (what we typically call 1080i and 720p).
Cool. So I wonder what it would look like if say a (hypothetical) broadcaster decided to transmit at 24fps (maybe to save on bandwidth or something) and my vintage (2001) HDTV with integrated 8vsb OTA receiver were tuned in on it. I would assume it would just be "converted" to the set's (native) 1080i (as it does with 720p signals) or in the case of standard - def (480i) supposedly to 540p. Obviously somewhat depends on the resolution of the signal, but I'd be curious. I can't even recall now if 24fps is supported in the lower resolutions.
Anyway, if the circuitry is indeed included in all tuners, I would guess (on this set at least) it should just appear as any other ordinary 24fps material would that were broadcast at 60 fields per second.
They've obviouly left a lot of "wiggle room" for various possibilities in the orginal implementation of digital broadcast standards. I also like that they've apparently been updated to include the newer compression standards.
With all of this then begs the question of the OP. I guess my answer would also be that there's no reason they couldn't do it, either, but there wouldn't be much noticeable inprovement for most folks, (esp. in my case with my old TV) and those that want it can at least enjoy 1080p/24 PPV channels already.
On another note, there's supposedly talk of theatrical moving toward 48fps! Might that be some kind of conversion problem, or does it fit mathematically like 60 fields seems to with 24fps? Hmmm..... Can't wait to see that on the big screen. Maybe we might live to be able to say goodbye to judder forever!