Most unit like MP5 for hostage situation, instead of M4 or
others, it is a submachine gun which is smaller in size and can
bring into action faster then longer rifles.
Next is the 9mm rounds, instead of 5.56 of a M4 , in hostage
situation you don't want a over power round to over penetrate.
Example go through a bad guy and hit the hostage behind, or bound
off a wall and accidently hit a hostage.
Also the close bolt and delayed roller of the MP5 ensure the first
round shot is totally acutal, with mininal gun internal movement
which can affect the shoot. As the first round is the most
important one, which usually does the kill.