I'd imagine the major constraint was the need to have a large inspection area on the US side. The area where the new crossing lands seems to be less populated than the Southfield area.
Exactly, on both sides it would be far more disruptive to residential developments, and require a lot more land acquisition on both sides. The Canadian side has already required over $500M worth of expropriations, routing it through LaSalle could have easily exceeded that (not mention the social issues that raises).
And, although they'd probably never admit it, building closer to the Ambassador Bridge makes it more likely that it will divert traffic from Maroun's bridge. Particularly with the 401 getting closer to the Ambassador with the new extension, if the new bridge landed at 75/Southfield a lot of traffic would use the Ambassador and brave the remaining lights on Huron Church Rd rather than potentially going 10+ miles out of their way.
Oh, there's no hesitation in admitting that the goal was to take traffic off of Huron Church Rd. As far as taking the traffic specifically away from Matty, I think that's just an added bonus.

But in terms of routing, that hits the nail squarely on the head. On the Canadian side the routing is able to serve both semi-local traffic via the EC Row, and traffic originating/destined in Windsor's south end and points beyond via the 401. A more southwesterly routing there would take that ECR traffic and keep it on Huron Church.
On the American side, a more southwesterly routing would attract the same trips heading to/from the south/west on I-75 and I-94, but traffic heading to the north or to I-96 would indeed be far more inclined to go to the north.
So in terms of southern traffic, yes, a different route would be shorter, but all of that traffic is still better served by this route than any other. But the traffic heading more northerly within Windsor and Metro Detroit, which is likely in far greater numbers, is far better served by this routing than that southern one.