system
Given the following post, respond as if you're commenting on Reddit:
human
[WoW] Arthas had an apocalyptic legion in Northrend. How didn't he have control over the island?

Arthas had control of an almost infinite, perfectly disciplined army with overwhelming biological warfare capabilities as well as ancient magic and technology by the Nerubians. How the hell did he let the Drakkari go rampant for so long, the Taunka, Kalu'ak and Sholazar tribes live for so long, not brutally put down the Nerubian rebels, not exploit the huge deposits of Titan lore and tech literally right next door (Wintergrasp was within spitting distance of the balconies of Icecrown Citadel!), and let the Scarlet Crusade create an entire cathedral right on his doorstep for the years between The Frozen Throne and Wrath?
gpt
There's a theory that a bit of the original Prince Arthas remained and that this shred of humanity worked subconsciously to keep the Lich King from reaching his true potential. Even if that's not true though, there are quite a few factors working against Arthas:

The Scourge *was* trying to stomp out the Nerubian rebels, but the Nerubians were both powerful and immune to the plague, so the fighting was a challenge even for the legions of the Lich King.

Sholazar was protected by Titanic magic that instantly destroyed any undead that tried to enter the basin, so he wasn't able to attack the tribes there. Undead are only just starting to advance on Sholazar by the time the Horde and the Alliance focus on Northrend, and that was only because members of the Cult of the Damned were able to sneak in and destroy one of the pillars.

The titan tech was protected by powerful guardians that even the Scourge might have had some trouble with. Even if they were victorious, since the guardians were not made of flesh Arthas wouldn't have been able to reanimate them to replenish his losses.

The Drakkari were also a pretty formidable force. Since they had stolen the power of their loa, they effectively had the power of gods at their disposal.

Also, on top of everything that was going on in Northrend Arthas was trying to dodge the attention of the Burning Legion as well as maintain his foothold in the Plaguelands, where the Argent Dawn was working with both the Horde and the Alliance to drive him out.