Either it didn't occur to him to dress up or he quite deliberately wanted to make a point of being himself.
It's probably safe to assume the latter as Zuckerberg has dressed up on other occasions - to meet President Barack Obama, for example.
The lead critic of Zuckerberg's sartorial efforts was Michael Pachter, a Wall St analyst for Wedbush Securities. "He's actually showing investors he doesn't care that much; he's going to be him," Pachter said in an interview on Bloomberg TV. "I think that's a mark of immaturity. I think that he has to realise he's bringing investors in as a new constituency right now, and I think he's got to show them the respect that they deserve because he's asking them for their money."
Pachter argues that Zuckerberg himself is the major risk factor in the Facebook model. The implication is that Zuckerberg is still a volatile kid whose youthful whims could drag investors down with him.
Pachter may know something we don't - but he does a pretty good job of coming across like a corporate dinosaur. His attitude is patronising on a number of levels. First of all, Zuckerberg might be young but he hasn't exactly spent his 20s backpacking around Europe.
He has been at the cutting edge of business for almost a decade.
Second, on Wall St, perhaps more than anywhere in the world, money talks. When you are the richest guy in the room you set the dress code. The smart brokers looking for the biggest allocations of stock should probably have turned up in San Francisco wearing hoodies.
And if at any stage while he was getting dressed for the meeting Zuckerberg wondered whether to dress up or just be himself, the logical response would be to ask: "What's worked for me in the past? Why change the script now?"
Pachter might want to ask himself who was the last 20-something to so totally conquer the known world? That's right - a Macedonian bloke who wasn't bothered if the Persians didn't like his sandals.
The hoodie is Zuckerberg's trademark as much as Steve Jobs' black turtleneck or Richard Branson's lion's mane of golden locks.
It is no secret that Facebook has plenty of knockers on Wall St. Anyone over 30 will still have strong memories of the dot.com bubble and bust at the turn of the millennium. The first wave of super-smart tech kids in hoodies did cost the bankers plenty.
Facebook is not without risk but unlike the dot.bombs of the last decade it has not been fast-tracked recklessly to IPO for a quick return. It has reached a scale that demands respect even though it has yet to turn its focus to pure profit.
Speculation about the prospects for this float has been more intense than anything in years.
A Bloomberg global investors poll released yesterday suggested the float is overvalued and demand is soft. Nearly 80 per cent of the 1253 big time investors, traders and analysts polled said Facebook didn't deserve its valuation.
But meanwhile, if yesterday's Reuters news story is to be believed, the same people are all in the queue for the stock. The news agency reported an inside source saying the issue is already oversubscribed.
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