A combination of:
1. Low per capita income
While not among the very poorest countries in the EU, Poland is far from being the richest. Per capita income in Poland is $12680, compared with $43660 in Germany and $9470 in Romania. This means Poland's per capita contribution to the EU budget is comparatively low.
2. Large population
Poland is much larger than other low-income EU countries. It has a population of 38.5 million, compared with 19.8 million in Romania.
Items (1) and (2) together increase Poland's eligibility for investment from the EU's Structural Funds and Cohesion Fund.
3. Large agricultural sector
Farms are eligible for funding under the EU Common Agricultural Policy (CAP), and Poland has a lot of farms -- although its agricultural sector has consolidated into fewer and larger farms since it joined the EU.
The CAP explains why France receives much more EU funding than the UK, even though they have similar population and income.