Sleep was banned because Sleep Clause was an abomination upon good tiering policy that only lingered due to inertia.
"We don't nerf mons, we ban them" is perhaps the most fundamental rule of tiering policy - OU didn't ban Rage Fist, it banned Annihilape; it didn't ban Last Respects, it banned Houndstone - and then banned Last Respects when it gained another user, so removing the move wasn't, policy-wise, nerfing a single mon to keep it in the tier.
The unstated corollary is "We don't nerf moves, we ban them," and we don't nerf mechanics anymore, which was one reason Tera Preview had pushback. [Weather setting ability] + [speed boosting ability in weather] would never be allowed in a current gen; the evasion boosting abilities would never be permitted under a "If you can't set the weather" condition, either. (The "But what if my opponent brings the weather?" problem did occur in Gen 5 and did cause further action.)
Sleep Clause was created before Smogon standardized policy, though, and it worked well enough in keeping sleep from becoming a problem that every generation just rolled it forward, but that's the key point - it worked. Once it stopped working, once sleep became a real problem despite Sleep Clause already existing and nerfing the status, then there was only one real choice. Why retain an exception to otherwise-consistent policy when the justification for having it no longer exists?
Thus, sleep ban.
Also, Darkrai wasn't the only sleeper causing problems; Iron Valiant could run Hypnosis with its fourth move, and Sleep Powder was springing up on Hisuian Liligant. In both cases, landing the sleep move and buying a completely free turn of setup - plus removing the best answer to the sweeper - could end the game on the spot with little recourse beyond "Hope it misses."