You asked why. Microsoft updates to Internet Explorer are known to but shouldn't have changed the email defaults. Let's not go into the discussion about this but move on.
You didn't supply what OS but in Windows XP you open up Control Panel, Internet options and find the tab where you select what program to use for email links. Change it to Outlook of your choice.
I have always used Microsoft Office Outlook for my default email delivery and, as well for sending information to either myself or to others off the internet. Now, for some reason, when I send something off the internet, up pops Windows Mail...I do not know why the change. As well, this morning two items coming to my email address came via Windows Mail. I do not want this but do not know how to correct it. Office Outlook remains my default email address so......I am really mystified. Is there a way to disabale Windows Mail without disabling the other aspects of MSN??????? Something must have been changed (not by me) and I do not know how to change it back.
PS: I have grandkids who use this computer so my guess is that they must have done something....