One obvious answer is that, at the time the diversion occurs - whether technical, weather-related or whatever - there is often no way to predict how long the situation that prompted the diversion will last, so it's only prudent to make alternative arrangements to get the pax to their planned destination by alternative means.
If after diverting, the weather then suddenly clears at the original destination, or the technical problem turns out to have a quick fix, well the bags have already been offloaded and ground transport ball is already rolling ...