Because they're likely cheap, and just because it's good at one thing doesn't mean it won't be good at another.
Don't we make the argument about guns all the time?
The MRAP is pretty good at defending folks from lightly armed, small ragtag groups of bad guys in an arid desert climate.
Can't imagine why HSI or CPB would want that...
Their construction makes them inherently dangerous to drive, prone to roll-over and brake failure type accidents. Collateral (read civilian) damage and casualties from these sorts of accidents in crowded urban and suburban areas will be substantially higher than, say, an up-armored suburban.*
Their size precludes them from a certain amount of maneuverability making it difficult to impossible to employ them in the tight confines often encountered urban or other built-up areas.
Their excessive weight precludes them from use on many roads due to damage to the paving or the utilities beneath.
The height and limited view through the armored windows makes it much more difficult for the drivers to see if there are people immediately adjacent to/in front of/behind the vehicle and will increase the danger of accidentally running over a pedestrian or a non-ambulatory or otherwise incapacitated person.
Certain parts subject to aging effects, such as tires, can not be effectively salvaged from extra vehicles and themselves carry substantial cost. The Michelin XZL tires for instance run about $1,000 each.
Due to their weight these vehicles suffer break-downs at a substantially higher rate than smaller vehicles that could fulfill a comparative role, hence the salvage supply of spare parts will be exhausted at an expedited rate at which point sourcing spares will become a much more expensive prospect.
Drivers of these vehicles require extensive additional training and then practical experience in the vehicle which equates to substantially increased costs. The time needed behind the wheel to become proficient drivers will obviate attempts to prevent vehicle wear through lack-of-use (and one must ask, what point is the vehicle if you're intentionally not using it to avoid maintenance costs?)
*As I recall the Army does not allow MRAP's to be operated on U.S. highways due to the increased danger they present. Instead they are shipped by low-rider semi-truck to the training facilities and back again. In contrast, the Stryker Armored Car is not restricted from the highways.
And finally, the ultimate reason: Because the police shouldn't have access to anything that isn't available to a private citizen.
