I asked the president of International Police Mountain Bike Association this very question and here's his knowledgeable opinion :
A MTB handles the urban obstacles encountered by public safety cyclist where a road bike would not (stairs, curbs, etc.). The riggors of carrying 50 lbs of equipment for the EMS Cyclist, mainly over the rear tire is another reason. This added weight when negotiating stairs and such would not be tolerated well by a road bike. So in essence, we trade speed for functionality.
I'd put Brunelle or any winner of the last (5) NYC, LA, London, Mexico City, Manilla, Lisbon, etc.,etc., winners against you or any other IPMBA instructor in a cat & mouse race anytime it could be arranged. The Alleycatter riding what he rode to win and the cop on his mtb w/ full gear in his uniform w/ full gear. > Jeez thats like putting a 900lb full dressed HD cruiser w/ saddlebags against a 220lb YZ/RM 250 in the same kinda race an actually being dumb enough to think the hog stands a snowball chance of winning. Wow the stupidity of that is truly mind boggling.
Great example:
https://www.youtube.com/watch?v=CWsMLmzfixw
That damn sure ain't no mtb he's riding!
I've never seen a clearer case of a poster who doesn't understand his own posting.
Bicycle police don't generally do high-speed chases, especially not on open road. They want a bike they can ride down stairs without the bike breaking down. A working bike is faster than a broken one just about any day of the week.