I've wondered this too. Upon looking into it, I think the confusion comes from a disconnect in what the Xel'naga plan was, versus what actually happened. My answer is a bit speculative due to how incomplete this lore is, though perhaps we'll get a definitive answer from Starcraft 3?
The plan, as you note, was for the "purity of form" and "purity of essence" to combine into what would become the new Xel'naga for the next iteration of the infinite cycle. This is understood to be the Zerg and the Protoss, as indicated by Amon merging the two into the hybrid species, which contained both attributes and would be the ones that actually ascended to Xel'naga.
Since Zeratul started uncovering Amon's plan to use the hybrid to destroy both Zerg and Protoss before ascension, giving Kerrigan the chance to take control of the Zerg again and for Artanis to unite different Protoss factions. Plus, the Terran helping out a little. They kept pushing him back until the only remaining step was to chase him into the Void and kill him as a Xel'naga. However, Ouros didn't have the power to do it himself. His quote from the top of this fandom wiki page gives us a clue: "You, the one called Kerrigan, can merge with my essence. Now, fulfill your destiny. Ascend as xel'naga. Continue the Infinite Cycle."
I have 2 guesses as to why this still worked.
I guess Artanis wasn't a pure enough form, however Kerrigan was pure enough essence. Either that, or Ouros didn't have the power to combine two different species, but did have the power to combine one with himself. If Ouros was of pure form and essence, combining his power with Kerrigan's should yield a being still of pure form and essence with the added ability of controlling the Zerg to help kill Amon. This is assuming that combining two species like this takes only the best traits from each.
Alternatively, perhaps the occasional Terran has purity of form due to psionic ability, but the species as a whole doesn't, which made them an unsuitable choice for Amon. Kerrigan fits both roles because of her psionic abilities and Zerg mutations. The difference lies in trying to get a whole species to ascend, versus an individual. Once again, the wiki page for Ouros states "Kerrigan needed to be remade on the planet Zerus, replacing her corrupted zerg essence with that of the primal zerg" and "Ouros implored Kerrigan to embrace her destiny and part in the prophecy by merging with his essence and ascend to become a new xel'naga, as she was the only one capable of holding the immense energy and power needed to defeat Amon and perpetuate the Infinite Cycle."
This is all a bit handwavey because psionics, gods, and merging species together is all ridiculous, so it may be a plot hole in the lore. This is just my best guess as the explanation, as the story ends so abruptly after the ascension event.