It would seem to me that the fable would be more accurate if the banquet being offered was not only the best food available, but the only food available. And if it was indeed the only food available, then requiring the potential diners to use a particular door is non-sensical. If indeed God is omniscient/omnipotent, then there is no reason for a single entry to maintain order. It would be ridiculous to assume that God cannot handle someone coming in the side entrance. There's no reasonable explanation, short of arbitrary fiat for the one entry rule.
And after all, isn't the important thing that we all go to the feast, and not the route we take to get there?
Why do you care why the rule is made, as long as it is a rule that is easy to follow.
G-d Himself will carry you to the door. Why do you care that He insists upon that door as long as He makes it easy for you to go through?
Shalom.