CME already had an exclusive deal with NYMEX to list metals on Globex. Rather than maintain the competing metal complex on ECBOT, which is legally dubious as well as expensive (maintaining two technology platforms) they sold it off. This will also appease the regulators regarding merger with NYMEX. Maybe bad news for traders, i would have prefered the liquidity in one place. Currently i dont trade gold futures because they are too thin for my style. Perhaps the NYSE will dry up and all volume will move to NYMEX eventually..
There would have been anti-trust concerns with having the gold & silver contracts from three different exchanges,(COMEX, CBOT & MidAm), under one roof.