Last or First?
I also wonder why Wisdom is the last virtue.
I suspect (can't prove it) that what Plato was talking about was the Saturn-esque wisdom gained through experience. If you have been a Judge, been Temperate and gone through experiences requiring fortitude, you can draw on all those in dispensing Wisdom (with a capital W).
The idea that Prudence is first seems to be coming from a different angle. I think it relates to a person who wants to be more virtuous. Imagine a reckless kid who says to us, "I'm sorry, I want to do better. How can I be better?" What would we say to them? We wouldn't say "Be Wise like your grandparents." We'd say, "Start by using your common sense...be prudent." We'd urge them, to use what they already have. What we all have. What the Greeks called "practical wisdom."
If I might use the perfect example right before us: imagine a poster comes here, reads something, it makes them mad, and they fire off a response. But then it turns out they totally misunderstood what they read. Too late, their rash and insulting response has other posters firing back; the new poster flees in fear, leaving the thread in chaos and ruined.... (Note: this is a work of fiction, not based on any real event
)
What would we have liked from that poster? Well, first, for them to have taken a moment to re-read and make sure they were right about what the post said. To have paused, thought about it, maybe gathered more information before deciding what action to take...to have been prudent. If they had done this, then prudence may have subsequently led them to the virtue of Justice, and deciding if what they'd written back was fair. And, if they'd decided it was fair, then on to the virtue of moderation, where they might have decided to adjust their tone. And finally to fortitude, where they would decide if they had the courage to post the response.
None of those other virtues can be gained if the poster isn't, first and foremost, prudent. From this angle, prudence being the leading virtue makes sense, at least to me.