Poland was not capitalist prior to 1945. Poland did not exist as an independent state during or in the century prior to World War II, and the country that controlled it immediately prior to 1945 was Nazi Germany, which was not capitalist. The countries that controlled it prior to Germany weren't capitalist, either. So it could not have "remained" capitalist in 1945 if it wasn't capitalist in 1944 or another recent year prior to 1945. It could have become capitalist, instead of communist, but considering how World War II ended, that wasn't really an option. Assuming some alternate history in which it had been an option, it still would have "become" capitalist, not "remained" capitalist.