Because from a practical standpoint (outside of the show), they didn't want to make a ST:TNG 2 with all of the old cast joining Patrick Stewart on a mission in outer space. And they needed a in-show explanation to do it. Would you have preferred an alternative explanation, like "everyone else is already dead?" or "everyone else is also in their 70's but still working on a starship somewhere else?" Are either of those explanations more believable or palatable? Sometimes you have to accept certain plot devices in order to suspend your disbelief and enjoy the show. Everyone has a different level of tolerance to these devices, but I think by and large what the writers did with this particular situation was acceptable, both in a practical and a dramatic sense.