Maybe because its cheap and easy, but I don't like this idea because if one string of three leds goes off then all the leds will burn.Manufacturers of LED lights measure and sort the LEDs into groups that have identical forward voltages so that they can be safely connected in parallel.
But this rope light probably has 3 blue LEDs in series and other strings of 3 LEDs in series with all the strings in parallel and they are powered from a single resistor.