They don't say when Poland will be eligible again, but green cards are intended to provide an immigration opportunity for persons from countries other than the main source countries of immigration to the U.S. The law states that no diversity visas(green cards) shall be provided for "high admission" countries, that is, countries from which during the previous five years there were more than 55,000 immigrants in the Family-Sponsored and Employment-Based visa categories. The list of countries, which do not qualify for the Green Card Lottery, is subject to change from one year to the next.
it looks like it's up to the government to decide one way or the other. if they feel too many people from one country got in then that country will be banned for the next year or two (or it could be longer) every country has a set number and those numbers vary greatly at times. yes, it seems very arbitrary but on a positive note they could always change their mind sooner rather than later. you just have to be patient and stay positive. (my family tried every year for five years to get a travel visa. seems like forever but it finally happened...)