Because the Polish Government cannot control Poles immigrating to the EU but they have a good relationship with the US Government.
That's the whole idea. Once the Poles are free to travel to the US, they will leave Poland. And knowing how they don't follow the rules about returning, they will stay here illegally, they will hook up with some Polish illegal company, who will pay them cash so they won't need a SS card and avoid paying taxes. Once they lose their job, they will be living behind a dumpster and drinking themselves to sleep. Sorry, but see too many illegal Poles living this way. It would be a disaster for Poland to allow Poles to travel to the US. I hope they never abolish the visas because that is the only way to control their population.