The reason for that is because you, as a seller, do not have a large enough history. Therefore, PayPal and Ebay have put a new policy in place to help prevent scammers from taking money without delivering the goods...or, vice versa. The hold that has been placed on your money is designed to prevent the buyer from being scammed out of their money...so that means, you must ship the item, and the buyer must confirm receipt, or no complaints must be filed, for you to be able to claim your funds. If the buyer submits confirmation that they have received your item before 21 days are up, you can claim your funds sooner; however, you MUST ship your item as soon as possible to be able to get the release on these funds. I know, I know, the buyer may have paid shipping costs with their payment; however, PayPal and Ebay don't care. You must ship the item first, due to your low seller rating.
Trust me, I think it's a rotten way of doing business too! Best of luck to you!