I don't think that's true. If I look at the NOAA tides on line site (see link) there's no dramatic difference between Pensacola and St. Petersburg. South Padre Island looks pretty much the same, too. When you get down to Naples things get weird, and continue to be weird around Key West, but by Virginia Key, the conventional semi-diurnal tide is well established. So the question should really be, "Why does the Gulf of Mexico not have a semi-diurnal tide?"