Because becoming more powerful =/= gaining a new ability instantly. With new strenght, one might open up to new possibilities, but you don't just gain it from gaining strength. The way Dante get it in DMC 3 make sense in the game because he fought a demon with this ability, then "absorbed" him, gaining the ability in the process. Vergil eat a fruit, became more powerful, but the fruit isn't related at all to the Doppelganger. That plus the fact that the Doppelganger on Vergil is a reboot idea makes for a unnatural introduction, and DMC 5 choose to not talk about it to avoid focus on that.
If for instance, Urizen was able to duplicate himself during the third fight, it would make sense. Even better, if Urizen gained the ability to create ghost-like reflection of his mind due to cheer willpower and the strength granted by the fruit, it would be even better since it wouldn't just be the reboot Doppelganger slapped on Vergil, but rather a new ability inspired from the reboot and something different from Dante's Doppelganger. That would make for a natural introduction to this new ability for Vergil. But instead, Vergil just pop it out of nowhere during the fight, and we never talk about it again. That's bad introduction for an ability and only create confusion for the player.