@rbp has a good answer. I'd like to add something to it.
For most aircraft, the horizontal stab provides stability and drag but relatively little lift. The wing provides 100% of the lift and everything else out there provides stability. And, of course, everything in the airflow (aside from the engines, which provide thrust) provides drag.
Contrast that with a canard, which provides stability and lift (and some drag, coincident with the lift). The canard typically provides 10 - 20% of the lift, with the main wing providing the rest. By putting the canard in front, and designing it to stall before the main wing does, the canard will be unable to lift the nose high enough to cause the main wing to stall. It's not 100% safe; there are still cases where a canard aircraft can stall but they're really obscure. Dick Rutan, who served as test pilot for Burt Rutan's canard-based aircraft designs, once joked that he could take one of Burt's planes up and try like to crazy to make it stall but "no joy; all I ever got was exercise."
Back in the late 1980s, Airbus started designing the tailplane to provide significant lift. After takeoff, they shift some of the weight aft (usually by moving fuel around) and take advantage of that. Airbus has been using this for over a decade to achieve greater fuel efficiency from their aircraft. With improved fly-by-wire flight controls, they've gotten to the point where they don't have to wait until after takeoff. The C-17 uses this idea, too (including the fly-by-wire). But it's my understanding that the tailplane provides no more than 10% of the lift of the main wing.
For the fictional aircraft, they wanted the aircraft to be able to hover. So they have main engines which can pivot downward. When in a hover, though, you need some lift forward and aft of the center of gravity (CG) to provide forward / aft stability and translation. Putting engines on the tail provides that. Making the tailplane a 1/2 span wing, with appropriate amount of lift, guarantees that approx 1/3 of the total lift will be provided by the tailplane. Which means that, when the engines pivot downward to hover, 2/3 of the total hovering lift is provided by the wing and 1/3 is provided by the tailplane. In this fashion, the plane is balanced in normal flight and in hovering flight.
I would've liked to see a large aircraft with the main wing aft of CG and canards, with engines on the canards (or maybe fuselage mounted, near the canards). But they went with a more "familiar" look; there's no heavy-lift aircraft out there with canards like I'm describing. The Tu-144 and Valkyrie both have canards, but nowhere near that large.
An F-35 has the tail of the engine pivot downward, aft of CG, and has a "lift fan" forward of the CG. A Harrier has a total of 4 downward columns of air coming off the engine when hovering, two forward of CG, two aft.