Actually, the history is rather like this. The Germans originally built the south carriageway as a 2 lane Reichsautobahn in the late 1930's, but the other carriageway wasn't completed. From Krzyzowa to Wrocław, it was built with 4 lanes, but in a rather cheap and poor way.
After the end of WW2, Poland simply left the A18/A4 as it was. The section from Krzyzowa to Wrocław was resurfaced in the 90's, but not done particularly well. That left the A18 in the current state.
In 2007, they reconstructed all the bridges on the route, as well as building the (missing) north carriageway. It wasn't resurfaced, but built completely new.
But apparently for the direction leading from Germany there was no budget. Some people say that the local council has decided to leave it the way it is on purpose.
Nothing to do with local councils, rather the GDDKiA. As the south carriageway must be completely torn up and rebuilt, it will involve essentially building around 70km of brand new road. The GDDKiA would rather focus on getting other connections finished first, particularly the S3 and S5 in Dolnoslaskie and rebuilding the A4 from Krzyzowa to Wrocław.