It's all very well saying riding a bicycle no-handed is easy, as if anyone on any bike should be able to do it. Yes, some bikes are easy - these usually have steeper head angles (e.g. many race bikes).
However, some are quite difficult to ride no-handed as the steering just flops to the side as soon as you let go - these usually have shallow head angles (e.g. many touring bikes). And if there's substantial weight on the bars, such as large light and heavier GPS, or a handlebar bag, the flopping to the side is likely to be worse. Having the headset too tight will also make it harder.
So it's mainly a function of head angle, fork trail and weight on the bars. Don't assume you can't do it, though, as it may be the bikes you've tried it on have had the kind of geometry or set-up that is not conducive.