Currently servers are currently paid minimum $2.13/hour. If they don’t make enough after tips to equal minimum wage over a pay period ($7.25/hour), then the restaurant is required to pay them up to that minimum wage.
Labor costs for servers, bartenders, and others caught in this legal loophole would have to increase by 7-fold to get up to $15/hour. Many restaurants and bars wouldn’t be able to afford that large of an increase without raising prices, given that many have a profit margin between 3-6% per several sources.
There have been some restaurants that have raised wages closer to $15/hour with varying success, but that hasn’t caught on widely yet.