To which "it" are you referring? If you happen to mean the former MGM - now Disney Hollywood Studios - it's because the two companies no longer have the agreement they once did.
If you mean the dining thing...
- Originally, Disney allowed actual reservations, but only same-day or, if you were staying onsite, up to three days in advance;
- For whatever reason, Disney changed from actual reservations to 'extreme call-ahead' seating, some time in the 90s I think, and originally called it Priority Seating - PS
- Because (apparently), despite the procedure being explained to the Guest at the time of the call, enough Guests misinterpreted "Priority" to mean that they had preference over anyone else waiting, Disney kept the theory but changed the name.
- For about 45 minutes one day (or 45 days one year, or something), Disney changed the designation to "Advanced Dining Reservations".
- Now, and for at least the last couple of years, Cast Members have called them simply "reservations".