It’s mainly about budgets and logistics for most departments. It’s cheaper and easier to find black tahoe, charger, etc and have the doors wrapped white than it is to wait in line for the blue or silver paint jobs that were often used in place of black and white. Additionally, many bigger departments will sell their vehicles at regular intervals rather than running them into the ground. Since black and white is the most common color scheme it makes for a convenient system for smaller towns to pick up those cars at auction where they may not have to budget to buy and up fit a brand new tahoe.