It was a crime of opportunity. It so happened that the President travelled by a building where Oswald worked, and he took advantage of that. Who knows how many assassinations did not happen because the opportunities weren't there.
As for who picked it, it was determined by the choice of location for the speech he was travelling to give. And that choice was mostly determined by the Governor. The problem that leaves is obvious. The Governor was hit and nearly killed in the same shooting.
The debate will rage forever, I guess, but on the whole, I think I agree with you. The most likely explanation is that it was the right guy (or is that the wrong guy?) in the right place at the right time. For Kennedy, I think it was a case of "some days, things just don't go your way....."
Hard to believe the goverbnor would personally pick the route for a motorcade. Approve maybe, select, no.