Betting on credit, and not having to declare and pay taxes on any winning will always allow for illicit bookmakers to operate. Plus most often there exists a personal relationship between a bettor and his bookie or runner.

Another allure is the ability to pay off the bet in increments, or in the alternative get a loan from a shylock to pay off the betting debt.

Legit OTB/or sports bet offices don't offer those services. So to a degree there will always be a certain allure for some betters. But for certain, bookmakers have been hurt by the increased competition in the market now.