Because the total system power draw depending on your setup can range from anywhere between 230-280W -
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Again, with an 80% efficient PSU the system would consume around 280/0.8 = 350W without the other components (monitor, case fans, HDD's, etc). Try playing a lighter game such as DOTA 2 or FIFA 14 and your system won't turn off during game (if vsync is enabled). Like me, you should be fine with an 800VA UPS.
Also, if I recall correctly, another member had posted it on this thread that there's a 90% efficient active PFC conversion involved. In that case, the power draw will be even higher - 280/(0.8*0.9) = 388W.