also the smaller airbus can be used on pretty much any (such as previously military) airport, where the much larger 747's are limited to major airports only.
If you're looking for cheaper tickets on a flight, you tend come up with a smaller airport, and thereby a smaller airplane (which also takes less seats to be sold, and so on).
btw, since nobody brought it up: it is possible to 'hack' an airplane from a suitable access point. Instead of going for terrorism, somebody really fucking up should be your first speculation. The uncontrolled descent with no warming sign gives the impression either the system was out or the pilots were (also keeping in mind that one suicidal co-pilot can single-handedly crash a plane too), which could happen for just about any reason. Over-fatigued pilot or heart attack for instance, while still handling controls.