You have to understand how TV works.
A show can get cancelled for any number of reasons, not always rating related. Show creators typically keep pitching idea even if they have a successful series because you have no idea how long that show's going to run, and you a) don't want to be out of a job, and b) the more shows you have on the air, the more money you make.
And Assignment: Earth wasn't the only show Gene pitched during Trek's run. It was the only one he was able to get produced in a fashion by doing it as a "baclkdoor pilot".
Heck, if TREK was struggling and facing possible cancellation, all the more reason to want to line up your next gig. You don't want to put all your eggs in one basket, or pass up a chance to reap the rewards (and financial security) of having more than one show on the air.
Look at all the failed pilots Roddenberry tried to launch in the lean years after TOS: Genesis II, Planet Earth, Spectre, etc. I'll bet he would loved to have had an "Assignment: Earth" series to fall back on instead.
How many different shows is Greg Berlanti producing these days? Or Shonda Rimes? Or Chuck Lorre?