Because you have to take a lot less of the car apart to fish the harness. You can get the harness without the connectors through much smaller spaces. I had seen a Youtube of the installation that they cut off the connectors at one end. I didn't see the sense in it after watching it, so I was going to do it without cutting the harness. It quickly became apparent I would need to take a lot of stuff apart to run the harness with the connectors. Butt splicing the harness back together is less work. IMHO.

Video here: https://www.youtube.com/watch?v=nIEKdYtIaWE