My guess is departments with an all-department fleet see a very high rate of vehicle abuse such as using official vehicles for personal business.
Having a mostly personal vehicle fleet flips the situation so the same vehicle is used for all personal business and also used for police business with reimbursement for that amount of use. Of course, the amount of official use can be abused as well by claiming more time/miles on their vouchers.
What I often wonder about is the cost of insurance. A commercial vehicle usually requires a higher olicy premium, so I imagine the same is true for personal vehicles used for police work. Is the department also picking up the tab for the insurance? Are coverage limits higher than the state's minimum requirements? The cops may be getting a fantastic amount of coverage at taxpayer expense whereas they might have only requested the minimum coverage to save money. I imagine deductibles are also paid if the vehicle was involved in an accident.
[I know when I lived on base in OK, I filed 2 claims for hail damage, and the Air Force paid my $500 deductible both times. A friend of mine was not living on base, but he was working late. Because he was not scheduled to work late nor ordered to work late, the AF tried to withhold his $1,000 deductible. He eventually got them to pay.]
No matter who buys the vehicles, the program they operate under -- subsidized or not -- is subject to abuse, and it all gets subsidized by our money.