Mass Effect depicts a universe where the galaxy is fractured, weighed down by past wars and feuds that still simmer to this day. The Krogan people suffered greatly when the genophage was inflicted upon them, reducing the entire race to tribal feudalism and general fatalism. Then there's Grunt, the perfect tank-born Krogan who was supposed to set a new standard for his people.
In some ways, Grunt accomplished his mission. He has the best Krogan genes possible, spliced together from legendary warlords of past eras, and warlord Okeer even imprinted some old memories to give Grunt context for his mission. However, Grunt cared nothing for Okeer's imprints and old grudges; he seeks only to prove himself, and that is both his strength and his weakness.
What Grunt Strives For
Grunt's tank was found on the war-torn planet of Korlus, and Commander Shepard claimed it when warlord Okeer was betrayed by the Blue Suns and killed with poison gas. Shepard freed Grunt from his tank aboard the Normandy, and it was clear that Grunt, aside from his superior genes, was a total blank slate. He had no interest in Okeer's personal grudges or politics, nor did he care for galactic politics at all. Grunt did have a vague hatred of the Turians and Salarians, but rarely did he act on those feelings.
Instead, Grunt longed to prove his strength, which is what he was born to do. He asked Shepard to ferry him to Tuchanka to get started on that quest. Shepard agreed, and Grunt began to immerse himself in true Krogan culture for the first time. The shaman escorted Grunt to the proving grounds, and after a good fight, Grunt became Urdnot Grunt. He returned to Tuchanka after the Collector mission to take some more crash-courses in how to be a proper Krogan.
Grunt was ambitious, driven and passionate about his quest to become the ultimate Krogan. To his credit, he aimed to be a disciplined and purposeful soldier, not a wild animal. Grunt became a child prodigy of sorts, earning command of the elite and clan-diverse Aralakh Company, a feat that made many other Krogan envious. Grunt was there to help Shepard rescue the Rachni queen in 2186, and he continued to serve as a soldier during the Reaper invasion and during the final battle for Earth. This is everything he ever wanted. But when compared to Urdnot Wrex, the great Krogan unifier, Grunt falls short in a few key areas.
What the Krogan People Need
Urdnot Grunt is among the best at what he does, especially given his youth and relative lack of experience. That is impressive, but no lab-grown genome or rifle mods can make up for Grunt's key deficiencies as Krogan warrior. He represents the very best of the wrong future for the Krogan -- a future where war is all they care about and slaughter is all they aim for. The Krogan are obsessed with feats of strength and respect among the clan warriors, but without a true future of prosperity and hope, all this firepower means nothing.
Grunt is not the future of the Krogan; rather, his elite genes and skills make him a tool of sorts, even after he broke away from Okeer's personal goals. Instead, it is Urdnot Wrex who laid down the proper future for the Krogan, something Grunt is there to protect rather than create. He is not a true leader.
Urdnot Wrex was inspired when he fought by Shepard's side, and he resumed his old dream of unifying the Krogan and giving them a future besides war. Wrex quickly and boldly began uniting the clans in the mid-2180s, and by the time the Reapers arrived, Wrex's project was at a tipping point. Even the most aggressive and conservative clans were becoming receptive to Wrex's new ideas, and Wrex was leading his people with hope, optimism and experience with how the galaxy works. A newcomer like Grunt couldn't have done this; he can only lead soldiers, not an entire civilization.
Fortunately for Grunt, it didn't fall to him to create this promising future for the Krogan. He doesn't even have the necessary experience with his own people, let alone with the other alien races out there. Instead, Grunt can act as a sheriff and peacekeeper of the new Krogan future, doing what he doest best while Wrex leads the way. Contrary to warlord Okeer's vision, Grunt did not create the Krogan future; instead, he is its protector -- and that is good enough.