> While my answer seems logically just as good to explain this stuff.

It sounds logical if you don't have a background in aerospace, but otherwise it's relatively inaccurate. For example:

> So far my best guess is that it's related to landing gear. Landing gear makes significant portion of the total weight

Landing gear makes up roughly 3% of the total takeoff weight. Hardly significant compared to fuel and cargo/passengers. [1]

> you want to keep it short and compact.

This is certainly true from a structural standpoint.

> Engines are the heaviest things in passenger planes, so landing gear is close to engines.

Compared to a person, yes. Compared to the total cargo/passengers, not really. The 787 MGTOW is ~500,000, of that, the two GE GEnx-1B engines weight about 26,000 lb combined.

The landing gear is "close to the engines" in your example picture, but this is because you typically place the main landing gear such that it is near the center of gravity. The nose gear only supports 8-15% [2] of the aircraft weight to make steering possible while taxiing. Some commercial aircraft have tail mounted engines such as the MD-80 (https://upload.wikimedia.org/wikipedia/commons/2/25/Allegian...). The wing (and landing gear) are indeed further aft since the CG is moved back further due to the engine placement.

In addition, comparing the number of wheels is a red herring for a commercial jet v. a cargo plane. The CG location relative to the wheelbase will be remarkably similar in both cases. However, military cargo planes often operate out of poor and/or shorter airfields. This limits the amount of weight you can put on each wheel if you're landing on asphalt rather than reinforced concrete, so you have more wheels with less load per wheel to keep from sinking into the ground. In addition, more wheels allows you to slow down quicker since you can spread out the braking action.

> With cargo plane the heaviest part of the plane is the cargo.

The cargo/passengers are a significant portion for commercial transports as well. Again for the 787, you've got around 100,000 in cargo/passengers (about 20% mass fraction). The C-17 carries 170,000 lbs of cargo with a 585,000 MGTOW giving a mass fraction of 29%. Not too surprising they have a higher mass fraction there since they're not adding any parasitic mass for things like passenger comfort.

lotsoffactors' comments on cargo loading/unloading considerations and the U-2 being a mid-wing aircraft are correct as well.

Sources: Aerospace Engineer and Raymer's Aircraft Design textbook (basically the bible of aircraft design).

[1] Chapter 15 of Aircraft Design: A Conceptual Approach (3rd Edition) by Daniel Raymer

[2] Chapter 11 of Raymer