Maybe it is because football popular in almost all countries and many people can play that game in many places. And when the world cup starts, all people's attention will go to that event and they will focus on selecting their favourite team to bet on while searching for the next winner. But betting without proper knowledge and knowing how to analyze the team will not give us a chance to win instead of losing the money.
One of the great advantages of soccer over other sports is that you do not really need much equipment to practice it, other sports require that you have some special equipment that is relatively expensive and that cannot be bought in many poor countries, but when it comes to soccer you only need the ball everything else can be substituted with free alternatives and you can still practice the sport, while in the case of baseball you still need a bat, many baseballs and gloves for everyone making it way more expensive to practice than soccer.