There is always a market for retro/nostalgia. It works when it's done well, as in GMK. And with Ultraman you have tons of material to draw from. Max had a lot of classic kaiju and seijin in new settings, and Mebius took us back to the Showa universe which many of us grew up with.
I do think that Max was the better show. But for the kid in me, Mebius helped take me back to the days when I raced home from school to watch the visitor from M78 fight monsters! And that's why I love it so much.
(Though I still wish they had done an episode with the evil space ninja Baltans
)