Clairvoyance assists?

Recently, I've been experimenting with alternative builds and summoner spells on supports and ran across an oddity. In playing around with the ability Clairvoyance, I was able to grant my top laner, a J4, vision of the half health enemy Rumble who was hiding in the bushes in top. Our J4 took the opportunity to jump on the Rumble, scoring a nice kill...But I didn't get an assist. I know clairvoyance isn't exactly a summoner used often, but I was wondering why the spell doesn't grant assists when an enemy is revealed by the spell and then killed.