Because of their anonymity. Often bitcoins are used to buy drugs and weapons. Also bitcoins are not subject to the state and are not taxed, a person can hide his income by withdrawing money into bitcoins
Yeah that may be a reason that it may be used for illegal activities but according to my personal opinion they still dont know about this mighty coin they are not in touch with the outer world and there are still third world countries out here where people think that it is some sort of silver coin thing so of bitcoin advertise itself on every country national channel then those countries will also use it for their benefits.