The asteroid 4864 Nimoy was recently named after Leonard Nimoy. It was discovered on September 2, 1988 so it went nearly 27 years without a name. Why was this asteroid chosen to be named after him? Were they trying to think of a name for it and chose to name it for Leonard Nimoy, or were they looking for an asteroid to name after him and chose that one?
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!