Probably because the email notification of submission, in many institutions, acts as a submission receipt and the email itself needs to be fairly accurately time-stamped to show the actual submission time, while notification of grading is less time sensitive and so can wait until the next cron job runs.
You also have the situation where some students, if they dont receive a success notification immediately will try to resubmit, will phone the helpdesk to find out why, will raise support calls, etc. So it makes seens to have the submission notification happen instantly. Their action gets an immediate reaction. The notification that someone else has graded their work (or released the grades) is not so time-sensitive -
I *think* the messaging cron job is set up by default to run every 30mins, so provided you have cron set up, then the messages should go out in a fairly timely fashion anyway (general advice is always have cron set up as parts of moodle - like this - do not function without cron).