I don't know why they don't build more flat rides, they are cheap to buy, easy to install and take up hardly any room.
My only guess would be because marketing seem to rule Towers, they probably think they can't market a flat ride unless its some kind of worlds first. But the public won't care, a new ride is a new ride at the end of the day. I would love one year to have a few flat rides open like Thorpe did with Slammer, Rush, etc.