There are a number of reasons I prefer IDPA, but one of the two biggest impediments to my playing USPSA is having to crawl around on my knees which are way too old for that nonsense.
I spent the first 5 or 6 IDPA matches just trying to run a clean match and shoot accurately, paying no attention to the timer. Once I got all the equipment issues ironed out and was not getting procedurals, I started speeding up.
Some of the other reasons...
IDPA attempts to do make you things in a way that might actually save your life.
You can actually be competitive in IDPA with a gun you already own (USPSA has become an equipment race)
The IDPA matches here are on Saturday (instead of Sunday like the USPSA matches - the other main impediment)