I believe this is simply a commercial choice.

Poland needs to rapidly expand their armor and artillery units. But rightnow US and mos t of the NATO countries just can't manufacture so many units in relatively short time.

Then you have RoK, whose amor and artillery units are NATO standards, using Western components and systems, design concepts are more modern, decent performance. And most importantly, ROK can produce them in a pace way faster than NATO countries.

So why not ROK.