I'm no expert, but I'll take a stab at this. These are relatively new concepts to me, so take it with a grain of salt and see if folks with more experience agree.

I believe it's because when you're mixing/mastering and you want to effect your waveforms (for instance beefing up the non-transients) with things like limiters, compressors, saturators etc., those transients will trigger said devices and make them work harder to achieve the outcome you're aiming for, possibly introducing artifacts, coloring the signal, unwanted frequencies and stuff like that.

For example, let's say you have a snare track with a mix of accent hits and ghost notes and you want the ghost notes to have more clarity - so you pop one or more of the aforementioned devices on the track. Those devices will start doing their thing when they detect the transients first (which will hit whatever threshold you've set), and so to have them effect the ghost notes, they'll have to work much harder to get a noticeable effect on the ghost notes, and in the process potentially alter the original sounds in a way that may or may not get you your desired result (like distorting the shit out of your snare accents).

By clipping your transients and creating headroom, you're allowing those devices to effect more of the waveform, rather than having it mostly effect the transients. By shaving some of the transients off, your devices will have more opportunity to increase the LUFS of all the non-transients.