The original characters are tested and still popular to this day. Although i don't think Shatner needs to be in this new film there are still calls for him to reprise the Kirk role. I don't think tng fans will ever be begging to see picard returned or data resurected.
My point being that the originals are still the most popular so it makes sense.
Other stuff like the dominion wars would be cool but not so much for a wider audience, even amongst trekkies.
Also the interaction between Kirk, Spock and McCoy has never been equalled so i think that is a huge reason to do a prequal