-Necrons did it (actually fits perfectly with the new fluff since wardcrons are bats*** insane and would perfectly do it for the evulz).
Except that Farsight was developed before the new Necron fluff.

Also I point out to the Krieg planet, that saw rebels siege nuclear weapons and turn the planet in a wasteland, and the IoM administration's reaction during the whole process was basically "we don't really care about you".
And that was the rebels doing it. And I quote Lexicanum:
Eventually, the Autocrats also began to resent the influence of the Administratum on the planet that they considered to be their own.
So it's not like the Administratum didn't care, it's that the Autocrats tried to throw it out.

And again: the galaxy is FREAKING big, so big that entire planets have to be used just to store and compute data. The administratum are probably doing a good job, given their circumstances. After all, they have to manage a million worlds, each different, while the lords sector, lords sub-sector and planetary governors all do their own thing, while often possessing sizeable military forces. The Adeptus Mechanicus also doesn't like to be bothered, and the Adeptus Astartes do whatever the hell they want.
On top of all this, communication and travel is slow and unreliable. How good do you think modern governments would manage?