The first reason not to ask the child is because they probably don't know the real reason. From their point of view, the answer is most likely something like, "Because the policeman won't let me live with my mom anymore." They are often confused because they get different stories from their family than from the social worker. Kids don't always get immediately removed from a home, and their parents will tell them all sorts of lies between when the social services investigation starts and the kids get removed.
The second reason not to ask is that it won't help. It takes a long time to fully open up, and the child doesn't know what information will be useful anyway. For example, one of my former foster children freaked out once when an adult who was preparing lunch innocuously moved toward her with a kitchen knife in her hand. That's not something the social worker told us to look out for, and my foster daughter certainly didn't volunteer it. Things like that just come out as you get to know them over several months.
That's why the previous foster parents are the best source of information about a child's fears, triggers, and behavioral issues, far better than the social worker. Good social workers know that and will arrange a meeting. If they don't, you should ask. If you're the first foster parent, you just have to figure out as you go how to best help your kids.
That's really no different than parenting your biological children. It's not like the doctor who delivered our daughter told us she would be scared of elevators and what to do about it. Parenting of any kind is always a work in progress.