Why don't Australia/Russia have large optical telescopes? Australia and Russia have large land areas. However, I am only aware of one 3.9m telescope in Australia. As far as I know, there are lots of mountains in Australia and Russia.

So why are there not many large optical telescopes?

The reasons for each country may differ because of their latitudes.
Optical telescope sites ideally need to be somewhat remote (to avoid light pollution) yet still accessible for construction, engineering, and observing; high in altitude; dry; with exceptional "seeing" (stable atmospheric conditions); and clear weather. Neither Australia nor Russia really have any sites meeting all those criteria: their mountains tend to have some combination of too low altitude, poor seeing, and/or bad weather. (This is also why there aren't any really big, modern telescopes in Europe.)

This site has some discussion of the different criteria that go into choosing the best sites for new, large telescopes.