There are a number of circumstances where you wouldn't necessarily want the executor and the trustee to be the same person.
The job of the executor and the job of the trustee are different.
The executor's job is to take care of the funeral arrangements, the body, the tangible personal property in the home. Cleaning out a home after someone has died is a large job and it requires a different skill set than managing money.
The successor trustee is going to manage the money, sell assets, do the distributions, it requires more financial skill than being an executor.