over 4 years ago - posila87 - Direct link
Explanation in FFF that KatherineOfSky mentioned: https://www.factorio.com/blog/post/fff-158

It is unfortunate we decided to drop the 32bit support just few months after releasing the game on Steam. It would not be reasonable to prevent access to the product people payed for, so we can't mark the game as 64bit only now. Doing that would make Steam Client update everyone to 64bit build of the game, and would prevent people from even installing the game on 32bit OS in the future.

If you bought the game recently and have only 32bit OS, you should request refund.