Although any one of them could easily step in to put a stop to their brother's machinations, doing so would likely be just as catastrophic as what the Rat King has planned.

Source: https://www.cbr.com/tmnt-armageddon-game-biblical-consequences-idw-publishing/

Characters like the Shredder and he Rat King have gimmicks that wouldn’t feel out of place in the WWE or AEW, so the two blend together rather naturally.

Source: https://screenrant.com/aew-tmnt-danhausen-brody-pro-wrestling-variant/

Even today, Krang has had his sights set on using the Technodrome to terraform the planet Earth, while the Rat King has looked toward it in the hopes of seeing the entire world burn.

Source: https://www.cbr.com/tmnt-armageddon-game-krang-dead-idw-publishing/

The Rat King is not necessarily one infected, but a conglomeration of multiple Infected likely made up of some of the original Infected in Seattle.

Source: https://www.cbr.com/tlou-every-infected-explained/

This event is known as and while the Turtles have assembled their allies - which shockingly includes the Shredder and the Foot Clan - so, too, has the Rat King.

Source: https://screenrant.com/tmnt-finally-kills-krang-classic-90s-villain-permanently/

While this concept and practice brought them closer than ever in their war against the Rat King, the experience showed each member of the Splinter Clan a different path for their future in which none of them were still together.

Source: https://www.cbr.com/teenage-mutant-ninja-turtles-splinter-clan-falling-apart-idw/