It would be very difficult to make a series of movies that would last around four-and-a-half hours or more total. Besides, there are some cases where once you write the original there really is no need for a sequal.
This is why there aren't very many animated trilogies...with most of these films one is all you really need. Apparently those "Land Before Time" people don't understand that. (but we've know that for years!)
Be sure to catch 24 every Monday at 9pm Eastern on Fox (and 10pm Eastern on Global). Watch it, because you know you're brighter than Michael Irvin.