It's a security constraint due to some phishing mails in the past. They probably just disable bitcoin withdrawals on accounts that multiple withdrawals above a specific threshold.

Mail to security@bitstamp.net and ask them to reenable btc withdrawals, if you can ensure your account is safe and you did not click or receive any phishing mails.

btw: I don't know why they can't just announce this on their new page. But that's just the way it is...