In addition to the ease with front wheel drives that other people have mentioned, it is also safer. When you back in to a space you have full awareness of what's around you in the car park, and are blocking the main driving route while backing into a place where no one is driving so are unlikely to have some speeding idiot hit your car. But when backing out of a space you lose vision on the driving route and are backing into it so you have a bigger chance of being hit by someone you can't see not stopping

While you can feel pressured by other drivers waiting while you backing into a space, it's far less pressure than when you back out of a space and don't know what's around you.

Similarly of you have a drive way at home, it's safer to back in to it as you have better awareness of pedestrians and other drivers versus if you are backing out of the space into a road.