Your question reduces to: why is gambling illegal? And the answer is that lots of people don't like it, and people are entitled to make things they don't like illegal.

A recent poll found that 55% of people think sports betting should be legal, while 33% think it should be illegal: https://www13.shu.edu/academics/business/sports-polling/uplo.... But if you read closely, support drops off dramatically in the 45+ age group, which is heavily over-represented at the polls, especially in local elections.

One study found that the median age of voters in mayoral elections in four cities was 60: http://www.knightfoundation.org/blogs/knightblog/2015/7/28/p....