Large trend-followers: why use futures rather than ETFs?

There are a number of large trend-following CTAs that have been successfully running for 10+ years. Their main instrument is diversified futures. Why not ETFs (is it due to liquidity / scaling, costs, risk)?

• There are some people trading ETFs in this manner, they are just not called "CTAs". Meb Faber for example. But they use less leverage than is possible with futures. – Alex C Jul 20 '16 at 11:02
• @AlexC wanted to use the "CTAs" here to refer to any large trend-follower. – A.L. Verminburger Jul 20 '16 at 12:26

Leverage: futures usually require much lower margin than their ETF counterparts. For example /ES (E-mini S&P 500 futures) requires about \$4K overnight maintenance margin per contract (may vary by brokerage) to control 50 times the S&P 500 index (currently valued at about \$108K). This is over 20:1 leverage. Furthermore you do NOT pay interest on your short positions.