the doctor can fix the chameleon circuit at anytime, he just chooses to leave it as it as as he likes it.
Also if they were to change it from a police box it would mean more money on costs for props/effects.
Not to mention the fact that the police box is kind of the icon for doctor who, so that will never change as its so recognisable. It's like coca cola changing from red cans or microsoft changing their windows logo, its not gonna happen.
Pity, as I think that a correctly working Tardis would have a lot more story opportunities than a non-correctly working one. Could think of a few really interesting story uses for it.