eh... simply because a tripod can stand on it's own, allowing you to take long shutter hence especially useful in night shots... a monopod is not going to do the job, no matter how stable you try to hold your cam for a long exposure, there is bound to be movements hence resulting in image blur...
monopod is more for support and is useful for ppl with big lenses / long zooms / indoor conditions... a monopod will help you restrain the shake to only 1 plane, hence resulting in less image blur...
TS, if you are unsure, get a good tripod, and those that are able to detach one of the legs to be a monopod de...