Why do almost all overwing exit slides open over the trailing edge of the wings, and almost none over the leading edge?

Question

Almost all narrowbody jetliners, and many widebodies as well, use overwing exits to augment their evacuation capabilities; to escape the aircraft, a passenger or crewmember in the exit row pops the hatch next to their seat, and then everyone else¹ chases them out onto the wing.

At this point, depending on the aircraft, they may simply slide down the flaps to safety, or a slide might inflate if the wings are too high above the ground. Either way, though, they’ll be going over the trailing edge of the wings if it’s a land evacuation;² even if the aircraft has multiple overwing exits per side (common among the larger jetliners), or if there’s a tail-mounted engine waiting to gobble up trailing-edge evacuees (like with the Fokker 70), everyone going out on the wings still evacuates off the trailing edge.

I know of only a few airliners with overwing exits that specifically direct evacuees over the leading edge of the wings, and two of these are very special cases:

• The Concorde’s wings extend for most of the length of its fuselage, so it would be essentially impossible to fit enough exits in without having some going over the leading edge.
• The Il-62 has a set of engines so close behind the wings that even the emergency-exit designers realised it would be a bad idea to send evacuees over the trailing edge (also, its overwing exits don’t have slides - instead, you have to slide down a rope over the leading edge of the wing).
• The DC-10/MD-11 family has overwing evacuation slides that go over the leading edge of the wing, as do some A310s, making these, so far as I am aware, the only subsonic airliners without engines right behind the wings to use leading-edge overwing exits.

For low-slung narrowbodies, where you slide down the flaps to get to the ground, the predominance of trailing-edge overwing exits is excusable, as the flaps are on the trailing edge of the wing; however, even for large narrowbodies and widebodies with inflatable overwing evacuation slides, the slides still overwhelmingly open rearwards, over the trailing edge.

Why are overwing exit slides leading off the trailing edge of the wings so much more common than ones that go out over the leading edge?

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¹: I exaggerate slightly; some of the passengers will likely lock themselves in the lavatories or run up and down the aisle(s) screaming like chickens with their heads cut off, and one or two might even use the exit doors at the front and/or rear of the plane.

²: Ditchings work somewhat differently, and some aircraft do call for occupants to fall off the leading, rather than trailing, edge in this case.

Answer 1

The special cases you listed (minus the very special case Concorde) hold the clue.

The Il-62, DC-10, and MD-11 are all tail-heavy due to the tail-mounted engine(s).

The emergency exits are designed inside-out, not outside-in, meaning, the locations are not only dictated by each exit's size, but by the cabin's seating arrangement first.

(e) Uniformity. Exits must be distributed as uniformly as practical, taking into account passenger seat distribution.
(...)
(4) For an airplane that is required to have more than one passenger emergency exit for each side of the fuselage, no passenger emergency exit shall be more than 60 feet from any adjacent passenger emergency exit on the same side of the same deck of the fuselage, as measured parallel to the airplane's longitudinal axis between the nearest exit edges. (14 CFR § 25.807)

The tail-heavy examples by design have the main wing more aft, leading to that scenario. The rest, the opposite.

The DC-9 extended family^[a] is more special, as there's an exit in the tail (also usable for entry):

Source: aircollection.org

And with the floor-level exits right ahead of the engines being not full-size (compared to the forward doors), it's been balanced that way. Otherwise it would have used an exit arrangement similar to the Il-62's, or the Vickers VC10 with a large exit forward of the wing in some configs, and/or over-the-leading-edge overwing exits in others:

Different VC10 configs; sources: wikimedia.org and wikimedia.org

The A310's reason is the original A300 fuselage that was shortened. If there are other configs, it would be related to passenger capacity.

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a: optional on DC-9, standard on MD-80/90, removed from B717^[1]

1: Becher, Thomas. Douglas Twinjets: DC-9, MD-80, MD-90 and Boeing 717. Crowood, 2002.