Why were the Spartans banned from participating in the Olympic Games for 20 years?

Because the Spartans violated the ban and started the war at the time of the Olympic Games. In this connection, they were suspended from participation for as much as 20 years. This was due to the fact that in ancient Greece they were very scrupulous about the Olympic Games, which were considered a universal event and a holiday for the whole of Greece, so it was even forbidden to fight at the time of its holding.