The defining features are size and cleanliness. A $100 multiplier on a 5,000-level index means one contract carries about $500,000 of notional-value, so a hedger needs far fewer contracts and pays far less in per-contract fees than with an ETF equivalent.
Add european-style-option exercise, cash-settled-option delivery and in the US section-1256 treatment, and you have a product with no early-assignment, no pin-risk and a blended tax rate. That is why index premium selling is dominated by these contracts.
Example: hedging a $2,000,000 portfolio against a broad index at 5,000 needs about four contracts ($2,000,000 ÷ $500,000). The same hedge in $500-per-contract ETF options needs roughly 40 contracts and 40 times the commission.
Related: index-option, xsp, am-settlement, cash-settled-option