The survey linked is what they found in the great pacific garbage patch, but the numbers are a bit fishy if we use them generally. The yearly amount of fishing net sold world-wide is as far as I can see is significant less than the amount of plastic dumped into the ocean each year.

That said, there are two large areas that contribute to ghost fishing. The fishing industry is getting centralized into fewer entities, and those might find it cheaper to discard nets rather than use costly manual labor to repair them or pay the cost of proper disposal. The second area is theft. A fishing net is basically an unguarded month of wages marked out with a bright buoy. I know fishermen that once they have a net stolen they will start removing the buoy for a while and simply drag for the net, which naturally increase the risk of loosing the net. If they can't find the net they don't even know if it is because they can't locate it or if it is because of more theft.