It's way cheaper to build ground-based telescopes than it is to build space-based ones (and you can easily upgrade it too). Thanks to advances in adaptive optics you can now get most of the properties of space-based telescopes with ground-based ones (even existing ones).

One major limit here is what the atmosphere absorbs.. so for X-Ray, Far-IR, etc. you will still need space-missions.. that's where the focus is now set mostly at ESA/NASA/etc..

Did you check out the SVG comparing telescope sizes that batbomb posted? The short answer is "you can build 'em a lot bigger" (or maybe "you can build 'em a lot cheaper" if you prefer).

These telescopes are gigantic and simply dwarf the James Webb Space Telescope. Adaptive optics have come a long way, too.