Two 6pins only deliver 150w. The move appears to be so that the connector can deliver 300w, hence the new 12-pin.
The 6 pin connector is intentionally limited to 75W but it could in theory supply up to 312W. There are two +12V wires, each of which could be rated up to 13A just like the wires of the Molex connector. That means 2x13x12 = 312W.