They are probably XP users. You can create a small notification on top of your website, notifying them that they should move to Firefox or Chrome because their browser is not safe anymore...
http://techcrunch.com/2015/04/16/google-will-still-support-c...
Firefox (as far as I can tell) still supports XP SP2, but who knows for how long.
You are assuming that they have a choice. A large chunk of them most likely don't.
The only way enterprise IT departments will upgrade is when they start having to take responsibility for security problems. As long as they can lie to their users and claim there's no need to upgrade, they will.