Because the jackscrew will not
fit "in the back."
Remember that the back end of a fuselage is a tail
cone - it is getting smaller and smaller with every inch you move backwards.
The jackscrew is about 4 to 5 feet tall. It needs that length to move the stabilizer slowly and precisely, and for the "lever arm" of a long thread to use the drive-motor power most effectively.
If you try to put that 4/5-foot-tall device at the back end of the stabilizer center section:
1) it won't fit anyway
2) it might fit if you impaled it through the APU exhaust pipe, which is in the bottom of the increasing-shrinking fuselage, thus eating into the available vertical space back there even more.
3) it will eventually get in the way of the elevator actuators, which
must be near the back of the stabilizer because that is where the elevators are.
Here is a cutaway view of a 737-800. You can click on the image and then click it again, to blow it up to 4K, and then scroll right and up to look into the tail cone.
https://thelexicans.wordpress.com/20...oeing-737-800/
Part "120" is the jackscrew (fills the fuselage almost floor to ceiling at that location), part "121" is the APU (about 2 feet in diameter, with an exhust pipe running all the way to the tail), part "117" is the stabilizer pivot, and part "109" are the elevator controls.
If you can figure out how to cram part 120 into the vertical space at or behind point 117 (don't forget to leave room for that APU exhaust pipe!), and without interfering with part 109, give us a drawing. You'll be one heck of an aircraft engineer!
