Is combat armour in YTU lighter than the TL8 stuff that's worn by troops from 1st-world nations now? It in itself is still heavy, especially after wearing it for several hours without a break.
Even more important, it's great at retaining heat. That's nice in cold weather (unless it's so cold you need CW clothing, in which case it's a pain to have to wear that stuff underneath) but hellacious in hot weather. There's no surer way to lose weight than to have to have that stuff on for days at a time in unpleasant arid locations in summer. In those circumstances I'd be giving players -DMs because of the distraction the sweat, and the fatigue it generates.
IMTU battle dress, being sealed and powered, is cooled, making it sooooo much better to wear in a lot of environments!
You're right, and my perspective it kind of counter-intuitive. But there is a logic to my madness.
I understand the heat issues from wearing body armor of various types all day myself, and you're right: it is like wearing a wetsuit under a wool uniform, and all the gear on top of that just makes things so much worse. Mainly, from my perspective and that of other people I know, the "more tiring to wear" part comes from having to wear a "full suit" of armor as opposed to being able to just wear components needed at the time.
For example, if I had to respond to a call requiring I also put on an assault vest loaded up with all the tacticool crap we had to have and its associated trauma plates, plus then put on a full gas mask because gas was going to be used, then I'd start to feel pretty constricted. It gets exhausting in various ways just wearing a mask for hours. MOPP was even worse - you felt you couldn't breathe at all and some guys freaked out from claustrophobia at various times in the suits. But at least police masks are more (slightly) comfortable.
And the armor is a pain to wear, and uncomfortable, but not so much as people think once you get used to it and how to wear it. And how to wear your gear with it. And move. And...oh, yeah, it's a pain, but getting shot is worse so...
I just assume that by the time the future 'arrives' combat armor will be lighter and more self-supporting as far as the load goes and so, more comfortable to wear and strapping yourself into a walking coffin.
I equate wearing battle dress to wearing powered MOPP gear in a lot of ways. The suit pretty much wears you, and while it is self-supporting, it is still fatiguing to wear it since you have to wear the whole suit - you can't just wear, say, the clamshell upper and helmet. I do assume IMATU that combat armor also has an exhaust and cooling system in it given the TL the stuff shows up in in the strange alternate universe Traveller posits.
The best TL-15 BD IMATU has a full virtual reality helmet display that makes it seem as if the wearer isn't even wearing a helmet. THat helps some from the psychological direction. But still, being sealed inside a moving sarcophagus for days at a time is going to have its own psychological and physical challenges for the wearer and in my book it will be an assorted of things that fit under a generic "tiring".
But, as always, your mileage may vary in your own games.