Because they are stupid and corrupt populists and don’t want to address their own corruption and love to make up stories of their brave fight against the evil EU.

Or in Detail: The government directed their farmers to increase grain output. And just when they did, cheap Ukrainian grain flooded their market and crashed the price.

The point here is that grain in the EU is more expensive because it’s produced to EU standards. Ukraine’s grain isn’t (for example using cheaper fertilizers and pesticides not allowed in the EU), so there should be no way to actually use it for most food-related applications in an EU country anyway, unless you violate EU standards.

Guess what happenend… corrupt guys at the top ignored the regulations to make a lot of money by dumping cheap Ukrainian grain into Poland’s market anyway.

Now the government has a choice. Prosecute the corrupt guys responsible (a lot probably directly linked to the government) and start working to ensure EU standards are kept up or make up a story how they have to ban Ukrainian grain to bravely protect their farmers from the evil EU willingly destroying their market by allowing Ukrainian grain.

The government are populists running purely on an anti-EU (and anti-Germany) narrative and there’s an election upcoming. You can guess the rest… And as with most right-wing populists all over the world, the rural population (including the farmers and farming communities) are their base.

PS: Poland as a country is very helpful to Ukraine. The government not that much. They are opportunists who took the chance to do something that looks good, then instantly tried to get all the help reimbursed by EU money. Also the support for Ukrainian refugees in Poland is limited with a lot being shouldered privately by helpful citizens and non-government organizations. The amount of money the Polish government actually provides is also (independently from the grain issue) announced to stop soon…