Willesden, thanks for an on-topic response.
In triathlon, the clothing changes are minimal, limited to footwear and a bike helmet. In pentathlon, the disciplines require complete changes.
The UIPM did float the trisuit concept for the new format - at least for the swimming and laser run - but it was a no-go for the athletes.
The swim is a 200m pool swim. Your time earns you points on the pentathlon scoring table. To get maximum points, you were a tech race suit which is not the same thing as a trisuit, which would earn you a slower time. A swimming race suit is unsuitable for anything other than racing - many swimmers don’t even put it on until after their warm up or warm up with only the lower half on. By design, the suit is very compressive and somewhat restrictive in terms of movement. The fabric is not durable and tears easily (you only get a limited number of swims out of a race suit - and they’re expensive).
But comfort aside, the bigger issue is seconds count and you’re not going to give up those points by wearing a trisuit.
RIding requires - at minimum - breeches, shirt, half-chaps/boots, gloves, helmet. (I tried to change the jacket requirement to hilarious effect but that’s another story.)
Fencing requires its own set of protective clothing, all mandated by the rules for good reason (fencing instituted very strict uniform rules after the death of a fencer at the world championships in the 1980s). It’s layers of protective fabric and a hard shell chest protector, a mask, a bodywire, etc. It’s all mandated/rated for safety and it does take some time to get in and out of your fencing kit.
If UIPM chose to eliminate the utterly dumb fencing ‘bonus round’, the second day would consist of riding, swimming and the laser run. That would eliminate one time-consuming but necessary kit change. There isn’t an athlete or coach in the entire world who thinks the ‘bonus round’ is a good idea but for some reason, the UIPM retains it. So you see pentathletes fencing for 45 seconds in a most half-assed way - they just want to get it over with because it’s not worth the one single point you get if you win.
UIPM does have sub-sports that consist or continuous run-swim (biathle) and run-swim-shoot (triathle) which are a different challenge and quite popular in some parts. But it’s not an attractive spectacle - running in your swimsuit with swim cap and goggles on is perhaps not the best look.