"Compared to Apple's Safari 4.0 for Windows released in June 2009, IE9's holding its own. Apple claimed, again without any external verification, that Safari 4.0 landed six million downloads on Windows in its first three days. Again, simple math would put IE9 ahead of Safari 4.0.
IE9 is not as popular as Opera 11 and Firefox 3.0, however. Opera Software in December 2010 claimed 6.7 million downloads for its latest browser on the first day. Mozilla claimed eight million downloads in the first 24-hours following the Firefox 3.0 release in June 2008."
Most organizations (those that would actually be aware of updates that are available outside of Microsoft Update) won't download IE9 until a full review process. Which is why they're likely still on IE6. Well, that and old software that requires IE6 to function properly. Firefox and Opera don't require vetting since they are not commonly used as plug-ins for software, nor are web GUIs designed to work with them (think Cisco web interface for their switches for one). This allows FF and Opera to be freely downloaded and ran from day 1 on home and work computers (where allowed).
As for the Safari 4 browser, that's due to it being bundled with iTunes (QuickTime is in there too if you click on the usual button), like most toolbars are stuffed into the likes of a DiVX (Ask Toolbar?) or Acrobat Reader (Google Toolbar) install. It's also set in the Apple Updater as well.
IE9 was likely NOT in Windows Update for the same reason IE8 and IE7 were not pushed out on day1. Likewise, Win7 SP1. They're giving system admins time to disable the update on their WSUS servers or at least vet the browser. MS caters to more than just home users you know.