I will add my 2 cents. I like both sports and both sports have many "unrealistic" moves you would not use in a real life situation. For instance in IDPA you are penilized for dropping a mag before you go to slide lock, you must reload in cover, and slice the pie on targets to name a few. None of these are germain to real life.
In USPSA having to run and shoot 12 targets with 2 shots as you see them as fast as you accurately can, reloading between segments of the course and shooting the targets in any order you wish is also not real.
Let's be real both forms of competition make you a better shooter. You learn a smooth draw, to shoot fast and accurately, handle jams, and become more relaxed shooting a handgun. Unless you are unbeleivably well trained you will revert to standing still and shooting the closest target in an emergency. An example would be the NYPD's 17% hit rate in shooting events.
If I would have to guess as to why initially IDPA is more popular than USPSA I would say the speed of the game. I have shot against masters in IDPA and rarely but sometimes came close to their score on a particular stage. That rarely happens in USPSA where masters truly "smoke" a stage. It can be very intimidating. My first USPSA match I was squaded with Chris Tilly and 2 of his buddies, 2 SWAT guys and a Grand Master shooting my S&W99.
What I tell all shooters is to shoot your match. Learn from it and what myself and other shooting friends do is pick someone slightly better and hope to beat them.