The store keeps your original email as your account name. This is registered against all your purchases as part of proof that you have paid for them and can use/play your purchases.
If you change your email you need to go into your account on iTunes. Go to Store on the menu and click View my Account.
Click edit my account. Leave the Apple id (your old email) as it is and change the email address section below it. Enter your password and confirm. Click done at the bottom. You should now be able to buy using your new email, but remember your account id will still look like your old email address.