The reason I would wait a year or two (depending on the individual) to upgrade to CA is becaue of experience in the airline system.
I have seen a few CA that upgrade too soon. They are not confident and that manifests its self in bad ways. The expeiance will make you cool, calm, and relaxed. All good traits in a Captain.
Think of a year in the right seat as attaching down hill. It gives you a great advantage.