Using this handy-dandy voltage drop calculator we can see that a phenomenon called voltage drop will decrease the amount of usable current as energy travels through the 50 feet of copper. If we had simply used one pair (out of four) in the CAT5, we would have ended with a pitiful voltage of 1.03v once your Pi is plugged in on the other end. Poo.

By wiring all four pairs in the CAT5, we provide four times the medium for the electricty to travel through. BEHOLD, the end voltage of 4.88v -- perfectly within range for the Pi, even while drawing as much as 2 amperes.

In the end, you could probably get away with a little more length and still stay in spec, but your wire/power supply might get a little toasty.

Use the calculator, as you should have all the information you need to calculate the voltage drop. You may even be able to make a longer cable with a beefier power supply, but overheating in the cable and at the power supply could become an issue even if the correct voltage to power the Pi comes out the other end.