I imagine it's due to the transom angle, PLUS the weight of the hull (additional trim angle may help with bow lift).

Incidentally, I DO NOT RECOMMEND TRIM EXTENDERS. They are DANGEROUS.

Instead, I'd recommend "Transom Wedges". Using a transom wedge (instead) allows your engine's trim angle to remain "within it's engineered design range".

Extending the "angle" of the trim beyond it's designed range can, and will, result in less "lateral support" of the engine between the transom brackets and the swivel bracket. This can result in undesirable handling characteristics, metal fatique or failure, and potentially, loss of control and/or injury to persons or property.

In other words... it's a "safety issue".