It helps if you have a relatively heavy front wheel.... like, a balloon-style tire. Road bikes now are all built much lighter, and the wheel just doesn't have as much gyroscopic stability.

Also it helps if the wheel is larger--as in, not-a-BMX-bike. I see lots of wayward teenagers riding no-handed on MTBs, but not very many who can do it on a BMX bike. (Of course, the BMX seats are usually adjusted down all the way too ...???)

For recumbent bikes it could be any answer--a few are easy to ride no-handed, many are difficult to ride hands-free until you get up to a pretty good speed (20-25 MPH or so). Some won't do it safely at any speed. And then there is a few designs that don't ever require you to steer with your hands.