Because there is a long queue for featured questions. GAG will automatically send you a message that says they received your request to redeem Xper points, and your question will be featured soon afterwards. However, it can take up to a couple of weeks because there is a waiting list.
Could take around 7 days before you get featured. It's a long wait, and it could be much better.
The featured question system is pretty shoddy, sometimes they want you to choose MHO before you're question has even been featured, sometimes you click on a featured question and MHO was already selected weeks ago. lol