Large telescope suspended from a balloon

It seems that a large telescope suspended from a balloon
above the troposphere would make pictures as good as the
Hubble pictures, but it would cost less than Hubble.

A simple tilt mirror can stabilize the telescope
image up to 15 arcsec (7.27x10^-5 radians).
Source: http://sd-www.jhuapl.edu/FlareGenesi..._4014_p214.pdf
This is sufficient stability to get the full,
diffraction limited resolution from a telescope
having the aperture diameter of about one
centimeter. A much more sophisticated system made
of the tilt mirror and a separate system stabilizing
the housing can improve the stability by at least
three orders of magnitude, which is sufficient
for a telescope having the aperture diameter of
ten meters.

I wonder why nobody is talking about large
(ten meters in diameter) telescopes suspended
on balloons. Do astronomers lack technical
imagination, or are there other technical
problems that preclude such telescopes.