[Kamen Rider Lucha]

I can't speak for the rest of fandom, but here's why I'm not as into Ultraman...

Disclaimer: these are just my impressions of Ultraman based on limited exposure to the franchise. I could be way off. If I am, please correct me and point out series that don't conform to my criticisms! I'll probably watch those shows!

1. Militarism: From what I've seen, Ultraman is as much about the Special Defense Force side-characters as it is about Ultraman and the Ultra-mythology. For me, being in the military holds zero escapist appeal, so I have trouble enjoying escapist fiction about members of militarized police forces (unless they're trying to topple or rebel against said forces). Of course, there are Sentai teams and Kamen Riders affiliated with military defense forces and whatnot, but those are the shows I tend to enjoy the least (with the major exception of Jetman, as that show is about civilians who are forced into the military by circumstance, and the corrupt members of Sky Force are often as antagonistic toward the Jetmen as the Vyram are). In general, I find that Toei heroes are (ever so slightly) more outsidery and/or anti-establishment than Ultra heroes. The Ultraman stories I've enjoyed the most prioritize the Ultra mythology over the Special Defense Force characters (i.e., the Zero movies) or are so old that the militarism comes off as quaint, naive, and benevolent (i.e., OG Ultraman).

2. Action: I'm way more fond of dynamic martial arts-style action choreography than I am of giant-sized, pro-wrestling-style brawls. From what I've seen, the action in Sentai and Kamen Rider is way more kinetic, athletic, complex, and visceral than the action in Ultraman -- which is more lumbering and slow-paced (though I'm sure it's very impressive in its own way).

3. Style: Tsuburaya tends to spend their money on different things than Toei does, leading to the shows looking very different aesthetically (different cameras, video/film stock, SFX, CGI, etc...). I'm more accustomed to Toei's style, so Tsuburaya productions look oddly jarring to me (obviously this would change with increased exposure to Ultraman).

4. Accessibility: The show wasn't as popular in the west during my entry into the fandom, so there were fewer fansub options for older shows. Plus, there were no new Ultraman shows for the majority of the time I've been watching tokusatsu. Obviously, Crunchyroll and a slew of new shows have made this reason obsolete in recent years.

5. Backlog: There are just so many Sentai and Kamen Rider (and Metal Hero) shows I want to watch that the idea of adding in Ultraman shows makes me want to tear my hair out. It's just too overwhelming. There are shows I want to watch, and I'm sure I'll enjoy the hell out of them, but I can't even consider them until I've gone through more of my Toei backlog.

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And... that's pretty much how I see it. Obviously, feel free to challenge my perceptions as you see fit. I'm sure a lot of them are wrong.