Pro Tools is more widely used among professional studios, and that makes it somewhat of a de facto standard. When some of your work is done in a home facility, it's nice to be able to take a disk with the session data into a PT studio, and start in with vocal backgrounds, string overdubs, brass section, or whatever you need the extra real estate for. It's like the old days when you hauled your 2 inch from studio to studio, except the CD/DVD is much lighter.