Hey there TCinTEX
The movie studios who own the rights to the movies set the time frames that determine when a title can be rented, and when they can be purchased.
Unfortunately these time frames don't mesh all the time, particularly for new release titles, so often a new title can't be rented as early as it can be purchased.
If you want to go deeper, check out http://www.vodprofessional.com/features/a-guide-to-pay-windows-for-tv-movies/