Why? Well it's the simple reason that Outlook.com is more modern and it will eventually replace Hotmail anyways. Right now it is not known when users will be forced to change over, but we all know that it will happen eventually. Microsoft is clearly sending the message that it wants you on Outlook.com.
Right now the switch to Outlook.com is optional. If you do decide to upgrade, you can currently also change back. I did a post a few months ago when Outlook.com came out on how to switch back.
Right now you can temporarily switch to Outlook.coma and grab an @outlook.com email address and downgrade to Hotmail's interface if you want. You can still grab @hotmail and @live .com email addresses if you want as well.
Since Outlook.com's release, I have wrote about it a lot on Techman's World. The button below will give you a search of all things tagged with Outlook.com. If you have questions on the new Outlook, drop a comment and I'll try to help you.