You raise a very good and important question, but poorly conclude the reasoning.
The motorcade's slow left turn onto Elm practically right beneath Oswald's position is indeed one of the best opportunities for an initial shot. Far better than the oncoming long shot down Houston after the motorcade turned right from Main street, but equally as good as the behind shots taken after the motorcade had passed Oswald's position.
Why you conclude incorrectly Oswald's reasoning for not shooting at this moment is because of two things: Chiefly, because the alleged Grassy Knoll position is one of the worst positions in all of Dealey Plaza to shoot from, and secondly because we presume that Oswald fully intended to escape. The direct downward shot at the corner of Houston and Elm is a very good shot for a suicide sniper not intending to evade capture, but is a dead giveaway for someone intending to flee Dealey Plaza, as Oswald very successfully did.
If you would like to see what I mean, I have a link for PC simulation abandonware for you that you may download:
Home Of The Underdogs host: 'JFK Reloaded'
It's a safe and virus/trojan free download and a compelling and meticulously-researched simulation that generated a lot of controversy when it was released. I think that after playing it using the various modeling simulations and virtual camera angles, you will agree with me about the absolutely rotten position for an alleged 'Grassy Knoll' sniper.
I just said that there was a possibility of a shooter(s) behind the fence. But you, me, and the cat up the street will never know. But it’s been nearly 43 years so who knows. It happened, it’s over, and life has moved on.
“The direct downward shot at the corner of Houston and Elm is a very good shot for a suicide sniper not intending to evade capture,”
Well if I wanted to escape, I would set up for a guaranteed one shot kill to a full frontal torso target AND NOT 3 SHOTS AT THE HEAD ONLY- GOING AWAY.