TL;DR - because the wire attachment point is usually to the rear of the center of gravity.
When reading the referenced article I realized it also allowed for an arbitrarily tightly stretched cross wire because the steel plate would bear all of the stress, not the art. That was something I had not thought of!
As it was at the bottom of the frame, it also moved the CG lower, which also helps to reduce the tilt, even if the CG remains a little bit to the front.
> When reading the referenced article I realized it also allowed for an arbitrarily tightly stretched cross wire because the steel plate would bear all of the stress, not the art.
I don't really get this, if the plate was at the bottom. I assume the stretched wire would be attached somewhere at the top half, so the rest of the frame would need to bear at least some of the stress.
Unless you modify the frame. Or screw into the top of the frame. Or loads of options that aren't taken for one reason or another because "the frame has a wire across it" is utterly dominant.
The tension on cable strung between two points is a function of the weight pulling down on it, and the reciprocal of the sine of the angle the cable makes where the weight hooks on. For it to be straight, the latter becomes infinite...