Complex sentence: This was due to the Polish Soccer League games-coaches of the teams did not want their best players to leave their sides earlier because in the 1930s, most Polish athletes were amateurs-they had to work on daily basis.
Split into simpler sentences: This was due to the Polish Soccer League games-coaches of the teams did not want their best players to leave their sides earlier. Besides, in the 1930s, most Polish athletes were amateurs-they had to work on daily basis.