Put it another way - a 10th level Wizard with rolled d4 dice averages 25 hp, with high average they get 30, a significant boost. A 10th level Fighter with rolled d10 hit dice averages 55 hp, with high average they get 60, a trivial boost. The low hit dice classes gain far more from high-average. I don't think it's a problem for Rogues, but Wizards are already extremely powerful (ironic how 1e players used to complain abour M-Us being too powerful, when they've just got more powerful with each iteration!) and high-average hp removes their one remaining vulnerability, the possibility of low hit points.