Why do index CFD’s have a much higher trading cost than ETF/Stock CFD’s?

Why do index CFD’s have a much higher trading cost than ETF/Stock CFD’s?

For example to trade US500 it’s going cost roughly $0.50 per contract/share (all spread cost) whereas a QQQ ETF is going to only cost $0.06 per contract/share (0.02 per side commission and a spread of 0.02).