Angular Momentum. You know it in day-to-day experiences: It's difficult to walk to the center of a merry-go-round ONLY when its spinning. All material from the molecular cloud can't just drop to the center because most of it will have at least some component of rotation relative to the center of the cloud. As it moves in, the rotation speeds up, inhibiting further infall.

Instead of falling directly onto the protostar, most the material from the molecular cloud builds up in an extended disk, first. The material in the disk heats up due to collisions, and thereby converts its ANGULAR MOMENTUM to thermal energy. This allows the material in the disk to move further and further in. We see these disks (shown in green) around very young stars called T Tauri stars. These T Tauri stars also show bipolar flows and jets (perpendicular to the disks, seen here in red).