I can think up the following reasons.

(1) We Chinese were not very smart and we never realize
changing gears accordingto speed could greatly
enhance performance
(2) Gears were too hard to manufacture and maintain
for us Chinese
(3) Gears would greatly increse the price to the
level out of the most ordnary family's reach.
(4) The Bikes with only one gear is better
for execising because at varying speed, you
need to apply different strength, which
is goood for exercising different aspect
of fitess.

I think the reason could not be (1) at least,
because even then National Biking Team use
bikes with cheangable gears.