Its hard to explain until you have to do it. But...
This one is basically a matter of geometry. SUVs and trucks have a longer wheelbase, and since (usually) only the front set of wheels turn, you naturally have a larger turning radius. This means you have to start your turns earlier to make it in. Since the back wheels dont turn, you are basically pivoting on your back axel. If you line the front of the car up with the spot, so that at full steering lock you will just make it in, the back wheels are still even with the car next to you. When pulling forward into a spot, the front will rotate in, but the back wont do much, so the side of your truck will pull in and hit the bumper of the car in the next spot.
Now, when backing in, you can align your back axle with the spot you want to enter. The fron swings around and you dont hafta worry aobut hitting anything on the side.
That didn't explain well, so maybe some really crude pictures will. The front of the truck is pointing to the right --->
See.... when pulling in your truck might hit the guy next to you. The isle is rarely wide enough to get far enough away to make it work.
When backing in, this isn't an issue . When pulling back out, the aisle is wider than the spot, so you have room to make the turn when pulling forward.
Maybe that didn't help at all, but once you have to drive a full size truck, it doesn't take long to realize you often HAVE to back in if you want to park.