Varus doesn't seem worthy of Ascension

"the Ascension ritual was highly dangerous and intended only for those near the end of their lives, those who had devoted their lives to Shurima and whose service was to be honored with Ascension. " - Azir bio.

Nasus: "The people of Shurima went into mourning, for Nasus was its brightest star and beloved by all. The emperor himself begged the priesthood for an augury. After a day and night of communing with the divine, the priests declared it the will of the sun-god that Nasus be blessed with the Ascension ritual."

The Emperor had to beg the priesthood to consider it and they spent a day 'praying' to the devine for it, this is after Nasus was loved by all and spent decades serving Shurima.

Renekton: "In the ultimate act of self-sacrifice and love, Renekton lifted his brother in his arms, and climbed the final steps, fully expecting to be obliterated in the process by the holy energies of the sun disc. He deemed his sacrifice a small thing to ensure that his brother would live on. He was just a warrior, after all, albeit a talented one, while his brother was a peerless scholar, thinker and general. Renekton knew that Shurima would need Nasus in the years to come."

Renekton was willing to accept death to deliver his brother to the highs of Ascension because Shurima needed him over just another warrior, even with again decades of service to Shurima.

Varus: "During the earliest stages of the war with Icathia, even though it lay far from that cursed place, Varus’ homeland was attacked. While other wardens abandoned their posts to join the defense of the outlying villages, he alone remained, screaming in anguish with every arrow he loosed—for he had chosen to uphold his oaths rather than return home to protect his own family.

Emissaries from the Ascended Host found him kneeling in solemn meditation amid the corpses of his foes. It was said that his cold gaze unsettled even the god-warriors themselves, and yet, in recognition of his noble sacrifice, Varus was offered a place in their ranks."

Protected a temple once, sacrificing his family and the other Ascended, even though they were unsettled by his 'gaze', invited him to Ascension. One battle. Just a warrior.

"The famous legend of Renekton’s Ascension revealed that a mortal did not have to be chosen by the Sun Priests, that anyone could rise up." - Xerath bio.

Varus didn't need this either apparently.

Xerath and Azir weren't worthy either but one being an Emperor forcing the priests to do it and one stealing it. Varus, killed some people. GJ, here's ascension.