Why did some belief systems become world religions?

Believe it or not, yours is TWO questions.

Mainstream beliefs, eg Judaism and Catholicism, are first of all beliefs, with very strong obvious basis in say a Moses or Jesus, and only then does the belief develop. In Christianity, Jesus said that our understanding of the Faith would develop. And culture and the life in which our faith is expressed, that too changes things. Your clothing, food, social customs are there and are acted upon by your beliefs. When you celebrate a religious feast with a meal that meal will vary greatly depending on whether you are a Southern US Cathollic, a Latvian, or an African Catholic. Same Faith but expressed differently culturally.