MS server limitations, stakeholders or ignoranc
Most companies that require MSIE only made the mistake of buying and using Microsoft Web server tech.
Some are Microsoft stakeholders. They sell, service or survive only due to their relationship to Microsoft. Many posters in this forum fall into this category.
However, a few just want to make their test cases easier and so ONLY test their sites with MSIE.
Finally, companies don't like spending money on check off items. In many cases, websites are just check off items. Something you gotta have, but invest as little as possible in. So they put something up years ago and haven't bother with it since.
BTW, exploit crackers love these sites. They are much either to inject Windows exploit code into.