Liquidity. There's enough demand for ETH-BTC that you can convert $30m worth pretty fast; for ETH straight to Monero, you would be in for a much longer wait.
Monero is also not quite untraceable with the defaults. It requires understanding Monero a bit, and spending days churning your coins. The idea that Monero is so unlinkable is horrible marketing and is going to get someone busted.