Here's why:
* "A soul worthier than I": This phrase suggests that Virgil, while a capable guide, is not the most knowledgeable being in the afterlife. There is someone else, more worthy, who possesses greater understanding.
* "Ascend the blessed": This implies a journey towards a higher realm of knowledge and spiritual understanding, where Virgil's guidance might not be sufficient.
* "Shall wish": This highlights that the journey is not predetermined, but depends on the individual's desire for understanding.
Therefore, the sentence implies that Virgil is a limited guide who can lead the way, but the true ascent towards ultimate understanding requires a deeper, perhaps divine, source of knowledge.