Lower fees. Also, "exchange competition" is a thing in US stocks, but not, for example, in the futures' market. And there's HFTs in futures too, so having a si

Lower fees. Also, "exchange competition" is a thing in US stocks, but not, for example, in the futures' market. And there's HFTs in futures too, so having a single exchange wouldn't "remove" HFTs (not that you'd want to).