The current competition regulations can be found here.

Immobilisation Any robot that is unable to move or demonstrate control may be counted out by the judges. This may be due to mechanical failure, entanglement with the arena or arena hazards, such as the pit. When the judges identify a robot that is classed as immobile, a 10 second countdown will begin. During this time should the robot free themselves the countdown will be stopped. Robots unable to move after the countdown reaches zero are knocked out and must not intentionally interfere in the fight. Robots in the pit at the end of the fight shall be deemed knocked out.
Out Of The Arena When a robot is thrown out of the arena that robot will be deemed knocked out. The fight may be stopped while the robot is made safe should circumstances require this.

Because it is basically impossible to clearly define when a robot is in the pit, it counts as immobilisation. This then means that robots passing over the edges (but that don't fall in) aren't out. If you get caught on the edge (because one of your wheels is over the pit) you get counted out. And those that bounce off the sides/ bottom/ other robots in the pit and come back out are still in. This makes it much easier to settle difficult to call situations when a decision has to be made straight away since wrong decisions often result in arguments.

The Judges Decision is Final Arguing with the judges may result in disqualification from the competition.
Whatever happens it is clear that the regulations must be consistent. For that reason all FRA competition is run to FRA competition regulations that have been agreed by a voting majority of the roboteers. Where events have run to variations of the rules this has always demonstrated problems and this only results in disappointment.

The short answer is that the shallow pit is a result of the arena design and not the rules. Event organisers are encouraged to design mechanisms to make the pit more effective.