Not really sure about this too. Perhaps, its due to safety measures..
To ensure the coaster is able to complete the loop yet not wanting it to apply too much pressure on the tracks. Maybe look at it using conservation of energy..
When the coaster falls from the slope, it does have KE. This KE is converted into PE as the coaster moves up the loop. To minimise centrifugal force by the train on the tracks, v needs to be small. If v is small, KE is small and it is wiser to let it regain a little velocity by allowing it to fall again, else if the KE runs out of the coaster, the coaster may run backwards and this may be potentially dangerous for the people on the ride. You need to compare the curvatures of the tracks where the coaster is when v is small. The larger the curvature, the more tendency the coaster will move in the direction of the steeper curvature.