I think the question is more about 'old' customers rather than 'big'. The reason most of the time is that the company had a bunch of apps/websites built in the

I think the question is more about 'old' customers rather than 'big'. The reason most of the time is that the company had a bunch of apps/websites built in the era of IE6/7 maybe even IE8, and they need to be able to run in compatibility mode. It is cheaper for them to stay with old IE than to have these apps re-written for modern browsers. Hopefully we're nearing an age where the cost of maintaining these old systems is starting to be more painful than upgrading, but I'm sure we'll continue to see old IE requirements for another 5-10 years.