That's right, because she was Shepard!
First, they had to confirm that Shepard was not under control. That prerequisite alone was difficult to verify, because she had appeared far too deep within enemy territory. Under normal circumstances, it was utterly impossible to reach her—not to mention that Mongal-Mongal infested nearly every planet around her!
Then, if they were to send reinforcements, they had to successfully reach the Xyala system. Only an Endless-class Super Carrier equipped with Forerunner technology engines could accomplish that, so they had to deploy an Endless-class vessel.
At the same time, they had to ensure that the hearsay was true—that the Mind-harp could truly block all energy signals. And even if it could, they still had to make sure it could genuinely sever Mongal-Mongal's control.
Furthermore, since the Mind-harp had last been activated on Planet Five in the Rafa system, they needed to make sure this divine artifact was not a one-use item. Then they had to find a way to recreate the same conditions and activate it on the distant planet of Mugg-Fallow.
The Mind-harp alone involved such a string of unreliable and utterly uncertain factors.
Not to mention Mongal-Mongal itself. Could its method of control be blocked? Were Mugg-Fallow's coordinates even accurate? How were they supposed to defeat the Rotten Star Ring?
Well, everyone now knew what the Rotten Star Ring actually was. The Chiss records on Mongal-Mongal had described it as well—it was a ring made up of tens of thousands of warships!
Those warships had been acquired from the countless species Mongal-Mongal had devoured and conquered throughout its history of tens of thousands of years, perhaps even longer. Naturally, they were all heaps of junk, and whether they could even move was questionable—but there were tens of thousands of them!
Mongal-Mongal controlled so