A weight limit for tandem skydiving is necessary for multiple reasons.
All your weight sits on your thighs during the parachute ride down. This can be uncomfortable if you are overweight for your height. Nausea, loss of circulation, and even unconsciousness under the parachute can happen.
Students sometimes have trouble lifting their legs for landing, making injury more likely.
The harness sometimes cannot be fitted properly or safely.
Larger passengers often have to jump with smaller tandem instructors, making their job difficult and their risk of injury greater.