The cynics among us might suggest that the cross became the symbol of Christianity primarily because it is an easy symbol to duplicate. Christianity, like other belief systems, seeks to propagate itself through the use of succinct messages (what might be called "sound bites") and easily reproduced symbols. This is especially important when one considers the multitude of languages and cultures in which Christianity has been introduced, and the large number of illiterates that have been taken into the fold, in the ancient past as well as in more recent times. A symbol that is readily identifiable and easy to duplicate is very important to a movement like that. My guess is that the stuff about an empty cross vs. a full cross, or about winged orbs being a more appropriate symbol, all came after the fact.

I mean no disrespect to Christians in saying this; I'm merely considering the communications and "marketing" aspects of this. Whether it's a cross, a Star of David, or the golden arches, the value of a simple and appealing logo should not be underestimated.