I checked out a Alan Rickman video and wow you’re right, a lot of people do find his voice sexy. I didn’t find it sexy when I first listened to it (Don’t care much for the accent), but I see the elements are all there nonetheless.
Referring to Chase’s article on sexy voices, Alan Rickman has:
Purr: that vibrating sound you make when you try to imitate Darth Vader. Very guttural and Sexy when done right.
Depth & Resonance: Not only does his voice purr, but it is deep and carries, similar to Morgan Freeman’s.
Boredom: Notice how his voice does not have much intonation to it? It’s the perfect balance between robot talk and baby talk.
Here’s the article on it
https://www.girlschase.com/content/book-excerpts-get-sexy-voice
-Micah