Because the first....three? Volumes had the benefit of being animated by Monty Oum, who is absolutely fantastic at creating high-speed fight choreography. This gave it a core that made the admittedly lackluster but largely inoffensive cast much more likable, because they were associated with all this cool shit. This as well as the interesting setting meant it took several seasons of utter shite to fully kill off the potential of the verse, which gave it unreasonable staying power. So basically it's riding off the glory of being the swan song of a great animator with cool ideas, and a promising beginning.