>>96820619
Here's the thing about the Tau:
They're not THE top threat the Imperium faces...but they are up there. They're 5th place, after Chaos, Necrons, and Tyranids, and maybe Orks.

And, to complicate matters, whenever the Imperium is fighting the Tau, they're fighting an elite military force. Chaos is a much bigger threat than the Tau as a whole...but much of their ranks consist of cannon fodder cultists that are worse off than local PDFs. You can entrust the Imperial Guard to handle a large amount of Chaos and save your Space Marines for the elite threats.

You can't do that with the Tau. Their regular army is elite by Imperium standards so if they're threatening you, you have to send a proportionate response, it's not something the Imperial Guard can handle on their own, which is why even minor conflicts with the Tau tend to involve disproportionate amounts of Space Marines and Titans and DAoT weapons. So while the Tau will never be nearly as high a priority as fighting Chaos, they tend to demand a disproportionate amount of high-grade assets to fight them compared to what you see in other armies.

>>96809812
Enjoy living with the knowledge that all of your descendants will be executed for being race traitors as soon as the Imperium decides to spare the Tau more than a millisecond of thought and wipes them out.

>>96791008
Then just believe that ranged weapons will kill them.