Traders are simple creatures. They go for the bigges money in the easiest way.
Make open the easiest mode and they will all come. Make anarchy trading far more profitable and the will come there to.
It’s not rocket science, it just design choices.
Okay - capacity of a cargo-only Cutter is 792t. Round trip times on a short trade route are probably about 10 minutes ... and you want a bit of supercruise time so there's actually some chance to interdict.
Passenger missions currently pay ~200MCr/hour, so you'd need trade profits of around 42k/tonne to beat that. That sounds pretty silly, but if you adjusted Palladium sale prices to what you can get for cargo missions, it would work out about right.
So, the traders are here. Unleash the pirates! Oops, now it's not the most profitable. One moment while we double the sale price and try again.
Raw profit is 84k/tonne, so the pirate can steal half of that and the trader is still making more money that they could elsewhere.
Next problem, stealing 400t of cargo (with an instance limit of 20t at a time) takes the pirate about 20 minutes. Now the trader can only make two trips an hour, so their trade profit is way down again. (Also, the need to keep pressing Jettison means they can't take a break)
Profit increases again, this time to a massive 250k/tonne. The trader makes two trips an hour, giving half their cargo to a pirate each time. Both professions now make about 200MCr/hour.
However, there's trouble ahead - the pirates have 400t holds, so if they just traded the supercargo themselves rather than spending hours slowly stealing it they'd make 600MCr/hour. So by pirating rather than trading, they're giving up 2/3 of their potential income. Rather than the traders making 1.2BCr/hour and the pirates making 600MCr/hour ... both are now making 200MCr/hour.
So most of the pirates ... a few dedicated souls aside ... switch to trading. Now most of the traders go unpirated, because there's far more traders than pirates. Trade profits rise further ... more pirates give up the job. Eventually, everyone just trades the pirate-attracting cargo.
(You can break out of this, but it requires the trader to actually make a loss on the cargo on any trip a pirate catches them, at which point they'll - in Elite Dangerous - just go and do something else which has a guaranteed profit instead)