There's many factors (in my opinion) that helped Chrome to get there;
• Google is the number one search site and if you're not using Chrome, a huge ads will appear on
top right of Google.com, plus it's anywhere on the web, in banners and other kind of Ads.
• IE is just a "catch program", they don't bring anything new in ages, what they do is implement
features that become popular in other browsers (like tabs, unified search and address bars, etc).
• Many new and basic users now search for alternative softwares to fit they needs instead of just
use the preinstalled ones.
• Chrome is just good, fast and have a really good UI, I don't use Chrome, I use Safari as default browser
but I keep it installed to test works and it's way better than IE (again, in my opinion).