Because the GFI is comparing the currents flowing in the hot and neutral. When you short the neutral to the ground, some of that current goes to the ground instead of the neutral, so the current in the hot and neutral becomes unequal.
The next logical question would be "But what if there is no load, so no neutral current?" The GFI will still trip, because it constantly injects a small test current itself to detect that situation.
If current is flowing through that neutral, some would be diverted to ground. That would cause an imbalance through the GFCI, tripping it.
Ahhhh... very interesting. I did not know that it tripped even if there is no load (i.e. no external draw on the hot)... now I need to get a paper clip (actually an insulated lead) and test it out... yup.. it trips it.
So cool... gives me new respect for GFIs and the engineering involved. thanks for teaching me that!
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