IMHO the reason why TOS was rebooted, but the TNG era (or later shows) have not been is basically the following:
1. TOS is culturally iconic, but sadly unwatchable to a large number of people. I know many Gen-X/Millennial Trekkies who simply will not watch TOS episodes (although they will watch the TOS movies). In contrast, people of my age cohort regularly do rewatches of the other Trek series.
2. The TOS cast was partially deceased, and otherwise elderly, when ST09 came out. Thus you'd need an entirely new cast to revisit the era with the same characters. At that point you might as well reboot. In contrast, every TNG cast member is still alive, in relatively good health (even though they're nearly all 60+), and still active as actors. It would be insulting to revive now with a younger sexier cast. Worse, it would trigger nerd rage about why they didn't just get the old gang back together again. Better to let the whole thing lie low for another generation.
Once again, why are we debating about what would have "crossover appeal?" As long as new Trek TV is in the ghetto of CBS All Access, it's only going to be hardcore fans who will shell out cold hard cash to see it. No one else will sign up for the free preview just because they're curious about this Star Trek thing.
Oh...and your very first sentence under #1 is it. That's absolutely the entire reason. Every other reason you wrote is literally in the margins. Bottom line is that TOS is a cultural icon. The spin-offs are good, well-liked, often highly enjoyable NICHE products. You don't reboot niche products in a gamble to revitalize a major motion picture franchise. You reboot major cultural icons.
And TNG and friends ain't that.