Use of space mainly. It's easier to top mount a larger turbo, and cheaper because to low mount you'd generally need a 1-off manifold.
I mean I've seen a GT40 low mounted so it can be done, there is just no room to work on it without major arse ache
Centre of gravity/weight distribution and all that shit, low mounting in theory would be better. For street cars it matters not.
And a front facing plenum, generally makes shit harder to work on (save for a plug change). When you split hoses under the plenum, you'll understand the pain that a larger FFP creates hehe.