Perhaps because the contraflow only goes to the I-10/I-55 split. So 3 westbound lanes + 3 reversed lanes get diverted down to 2 northbound and 2 westbound, a 50% reduction in capacity at a junction in the middle of a swamp. Right now some of that traffic can be diverted to low-lying old 51 or through LaPlace and Reserve, but it is kinda curious why they don't continue contraflow all the way to Baton Rouge.
Maybe because I-55 is a better highway North bound. I'm not familiar with old 51.