Until recently I thought I knew and I thought it was because of heat rejection being less than with just water (with corrosion inhibitors). That would result in a "slight" loss of power which in a locomotive can mean the difference between pulling 100 freight cars or 110, and over the entire operation that amounts to millions of dollars difference.

More recently I learned (still researching more on this so take it with a grain of salt) that it's because some of these big diesels (at least most of the ones used on locomotives) aren't designed to keep the coolant completely out of the lube oil and in fact there is some oil pollution. If a little water gets into the oil, they are designed to withstand that and keep on trucking. But if antifreeze gets into the oil, it seriously compromises lubrication.