Core Training

Why is it that traditionally people train muscle groups for sets of 8-15 repetitions (it obviously varies a little on the training goal) yet when it comes to abdominals quite often people do a lot more repetitions.

In general you gradually overload your training by adding greater resistance, modifying the repetitions or making it harder. So instead of adding lots more repetitions to an abdominal exercise make it harder.  For example add a weight to the movement, use a Fitball or lift your feet off the floor.

This will be far more effective and increase the benefits.