Just the way it is.

Why would anyone want NTSC video to run at 29.97 frames per second? What about HD video running not at 24 frames per second but at 23.98 frames per second?

It wasn’t intended that way. Before introduction of NTSC color North American television ran at true 30fps. The addition of color had to overcome a significant obstacle. No black and white TVs were to be left in the dark. The new color system had to be backwards compatible with the old black and white system.

This posed a significant challenge. For a set of very specific reasons that have to do with physics of radio transmission the frame rate of television had to be slowed down a minute amount to 29.97fps.

HDTV radio transmission is not bound by the same constraints that once necessitated slowing down of standard definition television rate. HD in North America runs at either 23.98fps, 29.97fps (also known as 59.94i) or 59.94fps for another reason.

HD non-integer frame rates have been devised in order to accommodate easy downconversion to SD. Downconverting from 30fps to 29.97fps is a tough proposition. It makes perfect sense for HD to run at either 24fps, 30fps or 60fps but as long as there is a need to downconvert to SD or use legacy video equipment integer frame rates are not going to be feasible.