Windows 7...
...was still released with IE utterly embedded in the operating system, and a lot of the less tech savvy users, for example people that will be using Win 7 on their first PC, won't know that there are alternative browsers out there.
In my opinion, users should click their internet link for the first time and be offered a list of browsers to use as their default, loosening M$'s hold on the market. Let's face it though, this will never happen.