Having grown up riding tradtional "women's" or double downtube bikes, I found it was nearly impossible to ride "no hands" on many double downtube frames, while it was relatively easy on many traditional diamond frames. So my guess is, the answer lies somewhere in frame geometry.

Nowadays I ride a Sirrus, with compact frame, that doesn't seem tolend itself to no-hands riding either, although the head angle is supposed to be around 71.5 degrees. Maybe it's the wheelbase?