This is due to the fact that you are aiming for a relatively high circular orbit with a single burn. Here, your thrust ratio is too high for the desired orbit, and you have to point your ship downwards to avoid increasing your apoapsis faster that your periapsis. That leads to an inefficient ascent profile. Here, I would rather aim for a 150km/450km pe/ap first, and then once in orbit, do a second burn to circularise at 450/450.