Why do recruiters tell you Civilian departments won't hire military police?

I've seen this first hand.
A lot of departments won't hire you since you are either
critical person meaning you can't be really covered
you are part of a shift force and not having a person for long periods of time can cause them heartaches ( OT , coverage issues for non ARNG/USAR)

I highly think its very dependent on what department you are applying at

Source I have a few MP friends that tell me its hard for them to switch departments due to their status in USAR/ARNG

That would be his opinion he is voicing, not based on fact. I know for our Hospital Security force, they will only consider MP's as equivalent experience and training for their 2 year requirement. So take your recruiter's opinion for what you think it's worth.

From what i have seen, more populated areas do not want bad habits from other places coming into their department, more rural areas just need broader experiences. so what he said is partially true. when it comes down to it, it is all about you bringing to them what they need just as any other job.