As others have said, it's part history, part physics, part aesthetics, and part hype. Twelve-string guitars with solid headstocks do put the 3rd and 4th courses pretty far "out" from the nut, so the "break" across the nut can be a problem, but 6-stringers don't so much.

Like people have noted, many "retro" styles have slotted heads and 12-fret necks, which not only give you the slotted head string-angle advantage, but typically put the bridge farther back from the soundhole in the body, which has an acoustic effect.

I've heard it said that the mass of a solid headstock with geared tuners produces a different sound that the lesser mass of a slotted stock with (usually) lighter tuners, so, combined with the effect of the bridge placement, a 12-fret, slotted-head guitar will sound different from a 14-fret, solid-head one, and some folks like that difference.

Among 12-string guitars, I used to pretty much hate the Martin 12s, with their slotted heads and 12-fret necks, preferring the Guilds, which had solid heads and 14-fret necks. When Martin finally produced a solid-head 14-fret 12-string model, it was nearly as good as a Guild, so the design was indeed the deciding point for sound (to me). Of course, this was back "in the day" when nearly the only choices for 12-string players was a Martin or a Guild (unless you hadn't the money, in which case you got a Harmony/Stella or a "real" Harmony -- a 12-fret Harmony Sovereign Jumbo, basically).

So there are sound reasons to prefer either kind, and many, many makers (now) to cater to those preferences.

Bob