Originally Posted by Picard Sisko
Its hard to take a flying police box (tardis) seriously.
But it's extremely iconic and instantly recognisable. You couldn't exactly change it for something more "bad ass" or serious. That would be an instant fail for the series. Same with the Daleks. You couldn't change their design now.
The police box is actually more associated now with Doctor Who than with the actual police, such that the police lost the case regarding who owned trademark.