A lot of them are afraid of getting into trouble. By definition, if you're undocumented, you want to stay away from the authorities and give them as little opportunity as possible to come look for you.
At the same time, it's the perception. Every time we talk about illegals, people have this thing in mind that these are people coming in to bomb our airports or do whatever. Every time we hear something from the United States, it's that all these people are going there for a 9/11, but those terrorists all came in with some kind of a regular visa.
So by definition, they cannot be a significant criminal group.