Because lots of business applications and certain critical websites require IE. They won't run with anything else.
If your business depends on those applications and/or websites, your employees must use IE.
And there are a lot of normal users who don't know there's anything else -- they assume that since IE came with the operating system that it must be the best one to use.
We used to have to use IE on a secure website that uses certificates, but I don’t think they require that now. I’ll have to check that out when I return.