Valid question self teacher. A broom handle could certainly be used like a bo in a pinch. But there are a few things that separate the two when it comes to real practice.

First, a broom handle generally has one rounded edge on the top and one edge that is either grooved or spiraled to accommodate a broom head. The difference in sides can be distracting and annoying during practice. Second, some bo's (like the one featured on this page) have tapered edges which create a center of gravity in the bo toward the middle. This can be valuable for spinning techniques. Third, the wood in broom handles is not as carefully tested as that in bo's because broom handles are not being considered for extensive use and abuse.