[pietjepuk88]

I think the main reason is that the (big!) ships just don't have room to pass each other anywhere other than on Gatun lake. They can't pass in the Gaillard cut, and not really in the approach harbors either. So it's one way traffic basically.

Also, anything taking the old locks out of operation for a few weeks/months is basically a no-go. That's just too costly.

As a side note, do keep in mind that water is already "saved" a little by going up in a convoy. With the three lock chambers, you get to reuse the same water three times. Not exactly true of course because convoys start and end, but you get the idea.