It's a combination of all of the extra bulk, eight part colors/materials and massive injection-molded flexible cloak as well as the globally increased price of materials (oil/hydrocarbons and therefore plastic) and manufacturing, shipping, and distribution (and even if this kit came out years earlier before the latter issues were compounded by things like COVID and rising oil prices it still likely would've been either in the $37-42 range or limited to P-Bandai). It's an unfortunate development across just about every hobby at the moment, and for all the rightful flak they get for anti-consumer practices (like making more new MGs exclusive to P-Bandai than released via retail each year for close to a decade now) they at the very least haven't ratcheted up their prices across the board as much as companies in adjacent markets (looking at you, Lego, Hasbro, Games Workshop, etc...)

From when this kit was first announced last year Bandai stated that they do intend to release versions of the others in the future, but they haven't released any more info since (but that doesn't mean they're not going to). If they're already at the prototyping stage for any of the three, we might see some uncolored test versions or even just cutout images shown off at this year's Shizuoka Hobby show in a bit over two weeks starting on May 13th, so keep an eye out for that (Dengeki Hobby usually covers all of the reveals at the event and takes photos of all of the booths over a few days so they're a good place to check).