Probably because the iPhones are more prone to damage, since people take their mobiles everywhere (whereas an iPad is less likely to move around so much, and therefore will suffer less damage).
People drop their iPhones in toilets all the time. When is last time you read of iPad dropped in toilet.
When last time you read someone sat down with iPad in back pocket.
When last time you read someone dropped iPad from lap getting out of car.
When last time you read someone washing iPad left in pants pocket.
When last time you read someone spilled their drink on iPad at bar or dinner table.
When last time you read girlfriend through iPad against wall because boyfriend texted another girl.