It seems Xdrive, and the Chrysler and Cadillac rwd based cars also puts the front driveshaft through the oil pan, Audi car quattro is the same layout as Subaru uses, therefore no inline 6 engines for them either, as the engine is in front of the front axle.BMW has X-Drive and Audi has Quattro, both produce sedans with traditional engines (non-boxer) and AWD. Dodge/Chrysler had(have?) an AWD version of the Charger/300, which is a large RWD sedan platform, I believe there have been others.
It's not that Subaru is the only game in town, it's that they've made it their only game, and have subsequently become synonymous with AWD despite Audi being at it for longer (Quattro has been around since 1980, full-time SAWD didn't come from Subaru until 1986).
GM has done some weird crap. Who else, with a mid-sized SUV, has gone this route? The WK and WKII both managed to do AWD with a variety of engine options without this nonsense, as did Ford with the Explorer. The Trailblazer and Envoy with this setup really had me like
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Many RWD based SUV's don't worry about CoG that much and keep things a bit simpler and just put the engine over the front axle and I think some of the Audi SUV's do this too.
The subaru and Audi car AWD setup allows longer front equal half shafts which in theory allows more suspension travel. The through the oil pan systems need to have the front diff beside the engine so the half shaft lengths are a bit limited.