No, it's definitely not lack of other time-occupiers with me. I've had intense interests when I was socially isolated, I've had intense interests when I was socially accepted. The intensity never really changed (the topic tends to, though, as when I'm depressed I get obsessed with depressing things).
From comparing myself with my NT (probably) brother, I'd say the big difference in our interests is that for him, it has to be social to be interesting. He's even torn himself away from a fun game to walk with me to the convenience store because the game is less fun for him if he's alone - even though I wasn't interacting with him before I decided to leave!
Of course, the funnest thing is when I can combine socialization and my interests. Such as when I visited my friend this past weekend and we watched Ted Bundy's last interview and chatted about the nature of psychopathy. My parents get creeped out when I talk about psychopathy, but my friend's a psych major too and she's interested in it.