I've been doing some digging around, but unfortunately there are so few extant garments from that period and the art is not very detailed, so I've had a hard time getting a handle on the "when", let alone the "why".
Marc Carlson's
Breches/Braies/Trousers page has some discussion of the etymology of braies, in addition to the extent trousers he could get information on in 2006. He also compares some possible patterns for braies, fully admitting that they are probably all wrong. The
wrapped braies idea is another interesting possibility, and looks very much like some 13th and 14th century illustrations of braies (especially
those from The Maciejowski Bible), but some illustrations look a bit more like baggy linen boxers, and they eventually turn into more fitted (probably cut and sewn) undergarments, like
these from
The Decameron).
The different theories on construction of braies seem like they might be relevant to the "why" part of the changeover. I've read where some folks have hypothesized that trousers and knee length hose (there's a discussion of vikings socks and hosen
here) gradually turned into braies and hosen - perhaps hosen allowed for showing off shapely legs more effectively than trousers, so the hose got longer and were pulled up over the trousers, which eventually became the light, baggy boxer-type garments some folks think braies might have been in the 13th and 14th centuries.
If braies are wrapped, though, that derivation of the garment doesn't makes as much sense to me. Maybe instead of coming from trousers, they came from wrapped Roman loincloth-type undergarments that I have seen called
subligar or subligaria or subligaculum. Interestingly,
this late Roman reenactment group soft kit description talks about "short bracae or femoralia of wool or [...] plain linen", and
this Wikipedia stub states "Femoralia or feminalia and tibialia were a kind of leg covering used in ancient Rome, the femoralia covering the upper leg (cf. femur) and the tibialia covering the lower leg (cf. tibia). Femoralia are sometimes described as short trousers or breeches, and tibialia as leggings." Unfortunately, I couldn't find anything else about "tibialia". If braies and hosen are maybe derived from Roman garments, it might make sense for them to be found in continental Europe before in England, and it would make sense for the Norman conquest to introduce them to England in that case, but there are a lot of “maybeâ€s and “ifâ€s there.
I went looking for extant garments to see if I could find anything that might help.
I found the silk hose of Holy Roman Emperor Henrich III, dated to 1056. They are long and fancy (like they might be meant to be seen), but I don’t see any obvious indication that they were pointed up like later hosen. I also found
the burial hose of Rodrigo Ximenez de Rada, the Archbishop of Toledo, dated to his death in 1247. Those look like they were pointed up to some sort of waistband, but I can’t tell if it’s a belt, which would possibly support the wrapped braies idea, or if it’s a drawstring casing-type thing, which would support the boxer-type braies construction. Frustratingly, I found an image of
“trousers made of fabrics imported from Spain in 1270-80â€, supposedly from Prague Castle, which really look like they could be braies, but I could only find that mention of that garment and no other information, so I don’t really feel like I can put a lot of faith in it.
So, in summary, I don’t know why the fashion changed from trousers to braies and hosen, but braies and hosen were worn throughout much of Europe from (at least) the 12th to 15th century, and possibly earlier. It seems possible that they were introduced to England by the Norman conquest, but I don’t really have any evidence for that. I can tell you from personal experience that they are comfy, though!