I think The Shat was afraid of being typecast while Nimoy was happy to be cast at all. Funny since Nimoy had an easier time of it immediately after TOS.
One of Nimoy's post-TOS projects I recall fondly is a made-for-TV supernatural thriller called Baffled
. And I also greatly enjoyed the series he narrated, In Search Of
, which introduced me to the mystery of the Oak Island treasure.