They're overwhelmed. Doing well in school and sports and cleaning their rooms is a lot to handle. Break big jobs into smaller parts, like having her put away her clothes one day and organize her desk the next.
They fear failure. When they were little kids, everything was new and making mistakes wasn't a huge deal. Now they're afraid getting something wrong will mean they're not as self-sufficient as they think. Reassure your kid that you care more about her containing the chaos in her room than keeping it spotless.
They lack motivation. Give her concrete incentives every now and then: She gets an extra half hour on the computer if she finishes her work. And don't forget the praise when she does the right thing!