I can understand that perspective, especially with an episodic show that lasted many seasons. It had ample opportunity to make some build-up.

However, I would say that also, with all new properties, even spin-offs of old ones, there is inherent risk and worry that there won't be more than one season. Often, you have writers trying to hamfist everything into that first season because they want their story actually resolved, even if it gets canceled. That feeling, on top of the difficulties of getting the series off the ground in season one, including a string of writers quitting because they couldn't get along with Roddenberry, and the cutting of a gay couple meant to be in the series, definitely speaks to the idea that the first season wasn't "in the bag" as it were, and there was risk if they waited, the show could get canceled.

This was also during a period where episodic television where one episode is completely self contained from all the rest was the norm and long branching stories between episodes really hadn't become the norm yet. X-Files would end up pioneering the coupling of a "monster of the week" format along with a larger story woven in.

Anyway, it's easy to speculate from here in the future, where we all revere the series as something amazing we grew up with. It had a troubled start, there were no guarantees, and it's hard to pin down why they chose to do things the way they did, especially with all the early infighting over writing.