Lt. Boma from The Galileo Seven was a really strong character compared to most single appearance crewmen.
I noticed that Lt. DeSalle, who did become a recurring face, was added in the very next episode.
The production had also just lost Grace Lee Whitney.
Is it possible that they were looking for a new familiar face in Lt. Boma?
As others pointed out, his hostile attitude toward Spock meant he was not going to be a regular (see: Balance of Terror
's Lt. Stiles).
Furthermore, Marshall shot the pilot for Land of the Giants
in 1967 (with the series finally airing in the fall of '68), so he--like many actors shooting pilots--probably hoped LOTG would go to series, as his was a lead role--the important note about that is such a role very rare for African American males on 1960s TV up to that time
, aside from Cosby on I Spy
and Robert Hooks on the original N.Y.P.D.
By 1968, he was well into production on LOTG and a semi-regular on Julia
, so his schedule was pretty busy (aside from a few one-shot guest appearances elsewhere), even if the Boma character was considered for a role upgrade.