Because Ajax *always* play in a 4-3-3 system. The fans demand it; they want attacking play. In a 4-3-3 system, there isn't really any room on the midfield for both Wesley Sneyder AND Rafael van der Vaart. Under the circumstances (Van der Vaart was struggling with his role as captain, would like a new challenge etc), it wasn't a bad move at all to sell Van der Vaart, earn some money off him, and really get the most out of Sneyder's capabilities.