Because of the multi-dimensional aspects of options. You can trade: direction, spreads, delta/gamma neutral, volatility, gamma scalp, sell premium, run a book, etc, all with limited risk known at the time of trade entry. It is also easier to write a business plan for trading options.
Instead of it being a 50/50 proposition when trading the underlying directionally (from a purely probability point of view), options gives the other side of the trade that many more ways to get it "wrong." Also, options can slow down the underlying, something that is often important for many that don't have the personality or the mentality of thinking fast on their feet trading the underlying.
Finally, the leverage is great, at the same time with limited risk. Nearly a perfect trading vehicle for the individual retail trader.
nitro
PS One desirable bullet point for me is that options trading is greatly aided by the computer and good [custom] software.