Why was this asteroid (4864 Nimoy) chosen to be named after Leonard Nimoy?
I'm not entirely sure, but I would guess that it's simply because the discoverer suggested it. After a few steps in the process of characterizing an asteroid's orbit, the discoverer gets to suggest a name (within reason) to a committee.  The full guidelines are available at the relevant IAU webpage.  (It's longer than is worth reproducing here.)

Note that there are plenty of "celebrity" asteroids, drawn from a wide range of human endeavours: Adamsmith, Balzac, Beethoven, Megryan, etc. If anything, I'm surprised that an asteroid wasn't named Nimoy sooner. There's already a Mr. Spock!