The main reason I can think of a 450 ceiling vs 510 is cost: a 450 ceiling is cheaper to produce and to sell (engines, wing, etc..).
I fly a 510 capable airplane, and I seldom reach 470, as it is only typically needed for operations close to max endurance with optimum levels to be flown. Most of my flying is between 400 and 450.
I only flew twice at 490, after cabin work and to check serviceability at different levels.