Why is the language so different in the South of poland?

I find the language around Norther Poland is not so difficult to understand, but the language used in the south is quite difficult.

why is it so different from standard polish?
which version is better? Northern or southern?

It's just different dialect... just like different versions of English, for example I've heard that it's hard to understand Irish people at first (for not native English) even if you think you know English pretty good. What about better.... well it's better to learn northern version, southern is reserved for highlanders.

My late husband was a highlander (from Sptycowice, SW of Krakow) & now when I get letters from his family & have them translated sometimes my translator has a problem putting it into english. I will also try to translate a bit of the letters with my Polish/English dictionary & I can hardly find the words. My husband did tell me that I do speak what little Polish I know with a mountain accent. ;-)
KT

Maybe you mean highlanders or native Silesians. The language spoken in Kraków or Wrocław is no different than normal Polish.

The area of Slask does have a certain German influence