Isn't it because if you need to move further than your rails allow and you already have an inline you'll have to have a set forward which will look ridic.
If the frame is made with some set back in mind and you want to go further than rails allow you can go inline...
Why are most bikes generally used with or designed for seatposts with setback? Seems more sinple to just design them around an inline post.
Would it be odd to get a custom frame made and then use an inline post with it?