Push bumpers add cost to an already expensive vehicle. These new police vehicles are not getting cheaper, nor are departments funding themselves flush with cash. A new Police Interceptor SUV (Explorer) is nearly $50,000. I suspect it's a cost-saving move.
For example, Chicago PD just announced they are buying 500 of those new Explorer-based vehicles at $47,000 each. If a push bumper adds, say, $2,000 to the cost of each vehicle, that's another million dollars on top of the already huge $23.5M price tag for those cars. Chicago is not exactly sitting on a huge cash surplus.
I can't even remember the last time I saw one on a Chicago PD vehicle. We still have tons of Crown Vics running around and they don't have the push bars, nor do the newer Explorers, Suburbans, or Tauruses. I occasionally see them on Illinois State Police vehicles, but their duties probably involve some different things than a cop in downtown Chicago would see.