Because pretty much everything that was possible to discover with earth based telescopes has been discovered. Sure, you can always do "science" with earth-based telescopes - reconfirming existing theories, making sure theoretical data matches up to real observed data, but there's a limit to how much "discovery" can actually be done.
Extra-planetary experiments like the deep impact mission allow scientists a glimpse at new experimental data that can confirm theories that there is no other way to confirm. Sure, you can observe a comet's tail to figure out its surface composition - but that has limited resolution and many elements simply can't be detected that way. This isn't even taking into consideration all the sub-surface elements that would be utterly impossible to detect from observation alone.
In essence, even if the Deep Impact mission mostly failed and scientists only gathered a limited amount of data, it is still very much worth its value when the alternative of building permanent terrestrial telescopes does nothing more than confirm data that has been confirmed countless times in the past.
> even if the Deep Impact mission mostly failed and scientists only gathered a limited amount of data
I don't think you meant it that way, but I'd like to clarify that Deep Impact's mission most definitely did not fail. The spacecraft survived for eight years after it completed its primary objectives in 2005.
"Because pretty much everything that was possible to discover with earth based telescopes has been discovered."
That's outright wrong.
Is the Large Synoptic Survey Telescope there just to look pretty and reconfirm existing theories? Why build the Giant Magellan Telescope when apparently there's no more discoveries to make? Or the European Extremely Large Telescope? The Thirty Meter Telescope?
Extra-planetary experiments like the deep impact mission allow scientists a glimpse at new experimental data that can confirm theories that there is no other way to confirm. Sure, you can observe a comet's tail to figure out its surface composition - but that has limited resolution and many elements simply can't be detected that way. This isn't even taking into consideration all the sub-surface elements that would be utterly impossible to detect from observation alone.
In essence, even if the Deep Impact mission mostly failed and scientists only gathered a limited amount of data, it is still very much worth its value when the alternative of building permanent terrestrial telescopes does nothing more than confirm data that has been confirmed countless times in the past.