18/03/2021
It's an interesting point...
The DeLorean time machine in Back to the future operates by opening a portal in spacetime in front of the car, which then drives through. You can see this happening in the film every time the car travels.
This is why the car has to be moving fast. The flux capacitor, with a 1.21 jigawatt operational power, can only create a portal that lasts a short time. And if you want to take your time machine with you when you travel (otherwise, how will you get back?), then you need the whole time machine to get through that portal.
So how long does the portal stay open? The clue is in the oddly-specific 88 miles per hour requirement.
A DeLorean is 168" long, bumper-to-bumper. That's 4.267 meters. 88 miles per hour is 39.3395 meters per second. This means the car can pass entirely through the portal in about 0.1084 seconds.
So basically, Doc Brown's time portal mechanism can only create a portal that remains open for 108 milliseconds. And the only way to get the entire DeLorean through in that time is to be going 88mph.
That's my official head-canon and I'm sticking to it.
EDIT - corrected my math thanks to posters pointing out the error.
