Because most people aren't geeks and just don't care about their computing environment enough to make the huge move of seeking alternate employment. Not to mention that in this lousy economy, your average person is probably quite short on options.
Besides that, how well do you think a site would do if it told you to find a new job - are you going to find a new job, or a new site?
I don't think the likely corporate users of ie would be programmers. Rather, they would be accountants seeking occasional entertainment on the internet.
Besides that, how well do you think a site would do if it told you to find a new job - are you going to find a new job, or a new site?