They didn't, and your question is a great insult to the men who did all they could to protect him that day.

ETA: Would love to know name of film you watched. I really enjoy conspiracy theory even though I tend to generally take with a grain of salt. I believe the reason that the SS weren't on the back of the car was because Kennedy himself ordered them off. There has been quite a bit of discussion about one person saying he did and then changing his story. I don't believe we'll ever know the truth, I do think there was something more, but I don't think saying that the SS "abandoned" him is accurate in any way. Not his personal detail.

If you want to know who ordered it, then the rumor (that I don't necessarily believe) is that it included:
Prescott Bush
Averill Harriman
John Foster Dulles
Lyndon Johnson
George Bush Sr.
Haroldson L Hunt
Roger Lewis

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