In my opinion, the Jedi Knight games are poor references in this case. In those games, as in the The Force Unleashed games, Force Push is a satisfactory game element. They were designed to be fun, not realistic. Think of the sabre combat in those games; both you and the enemy gets hit by direct lightsaber strikes multiple times. In the 'real' world, even a small and brief contact with a lightsaber's enery beam has the potential to end the fight, look at Obi Wan being hit in the arm in Attack Of The Clones.
Both Jedi and Sith are taught very early on to defend themselves against attacks through the Force. For example, have you tried using Force Push against the Jedi bosses in The Force Unleashed? If you have, you'd know that Force Push is pretty useless, even though arguably being the best way of attacking enemies in the rest of the game. You just waste 'Force Points'. Using the Force requires concentration, even in calm circumstances. Imagine then, trying to gather the strength when engaged in deadly combat where a tiny mistake can lead to your death. Even if you could, if your enemy successfully protects himself (which he should), you just wasted energy.
But, are you sure Darth Maul doesn't try?
Look from about 0:50 in this clip
. A brief pause in the fight ends with Obi Wan looking like he's been hit in the head, and Qui Gon seizing the opportunity to lunge forward. Couldn't this be an attempt from Maul to push Obi Wan off the platform? This would go along fine with my earlier points, Obi Wan could defend himself and Qui Gon realized that such an attack would leave him vulnerable.
The screenshot you provided is right after that event, Darth Maul could simply have realized that another Force Push attempt would at most kill one of the Jedi, and then he'd have nowhere else to back off to and the other would incapacitate him.
And finally, Darth Maul does!
At the end of the fight, when Obi Wan is blinded with grief and (..anger?), he is not as calm and composed as he should be, and he is unable to defend himself against the Push that almost had him falling to his death.