While that is true this happened there is literally zero relationship to lesser liquidity. In US markers retail does not drive market liquidity in any meaningful way. And the free money over the past 15 years has provided all medium and larger players with ample funding to make margin levels completely meaningless to them.
The reason liquidity is so poor in stock index futures is due to the quadrupling of stock index futures margin. Just 3 years ago margin for 1 sp500 Emini contract was $3500.00 now it's $12k just to hold overnight. Some brokers charge much more. Millions of small traders have been priced out of the markets.
Just like first-time home buyers are priced out of the housing markets.