I'm not really interested in his personal life,
In all seriousness, perhaps Cruise's interest in sci-fi is related to his beliefs in Scientology and that -sci fi plays much into their belief system related to Xenu and the end of civilization?
Just a guess.
Regarding his stature in modern sci-fi - I'm not sure he's any bigger than Zoe Saldana, Harrison Ford, or Bruce Willis. And with SW coming up - and if Fisher, Hamill and Ford possibly showing up again - that will eclipse everything else out there.
Cruise makes better choices, his BO grosses may be comparable with some of those (Ford hasn't had a hit in years) but the quality is better. Zoe's two scifi projects(ST09 and Avatar) make up $3.1 billion of her popularity.
Well Scientology itself is science fiction yes, but he never seemed to have interest in converting it to his films. Minority Report came out in 2002. Over 20 years of stardom later.