First of all, we need to get one thing clear - IE8 is dying! It's dying slowly and we should feel good about it!
Big companies rely on specific audience, in most cases other enterprise corporations and institutions like hospitals, academic institutions etc. Performing a major OS, browser or hardware change in most cases will be very costly and problematic - they need to insure that every computer on the network works as expected, which leads to even more support expenses. Also, most of these machines are locked down and restricted from installing any kind of custom software.
In the past Microsoft did not play well with the web standards and they implemented a lot of non-standard features in their browsers. MNCs spend a lot of money on developing browser-specific features that work only on older versions of IE.
That said, I see more and more companies transitioning to newer OS and browsers. Microsoft is dropping IE8 support January 2016, which is expected to end the hideous era of non-standardized browsers. Some months from now, this probably won't be a problem to cope with.
Good times are coming, we need to wait a little bit more.