It's easier to build and modify a telescope on the ground. Once a telescope is launched it's extremely expensive to go make any changes to it, on the ground new

It's easier to build and modify a telescope on the ground. Once a telescope is launched it's extremely expensive to go make any changes to it, on the ground new experiments can be added later. Currently we have no way of doing a Hubble style modification even if we could get to the satellites (JWST is going to be launched very far out for example) that was something pretty unique to the Shuttle program. On top of that building a telescope with a very large mirror is much harder because they have to be launched. The mirror on James Webb is only 6.5m (ish) and it requires a folding mechanism to fit inside the faring of rockets. Making a 30m mirror may not be possible without going to farings larger than any rocket around today. BFR if it actually pans out will make much larger space telescopes possible.