Ice melts faster in diet soda because of the absence of sugar. Regular soda contains large quantities of sugar in the form of fructose and sucrose.
Corn syrup, which contains sugar that helps to make the solution denser and lowers the melting point of ice, is usually added to regular soda, causing the ice to melt slowly. The temperature of the sugar solution in regular soda is lower than that in diet soda. Diet soda has a slightly higher temperature because of the absence of sugar, making the ice melt faster. The freezing point of any liquid depends on the density of the solution.