Writing back then was a lot less tightly controlled and the arcs were a lot less clearly defined. You had the Black Guardian thing during Davison's period, and the Key to Time during Tom Baker's but otherwise...yeah, it was just a 6-episode or 4-episode or sometimes 2-episode bit of stories (episodes being 22 min back then).
As such, I think it was a lot less likely that a companion's death or absence would be felt long-term. In addition, with Adric specifically, the production team (in DVD extras -- which are almost always good), discussed how Adric was never clearly defined. They had a character bio, but the charcter himself was never really effectively developed. The scripts for him were very erratic, so, like, in one episode he'd die rather than help the villain because he's loyal to the Doctor, and then in the next, he's all "Oh, you have a lovely idea here for ruling the universe. I'll help you out because it's logical."
It just didn't make a ton of sense.
The flipside of this is how they DIDN'T develop the Sixth Doctor who was initially supposed to just be erratic and cranky -- but because he was suffering from a wonky regeneration. Eventually, he was supposed to get darker and more calculating....but it never happened. They just kept writing him as cantankerous and snappy until the very last season with the Trial of a Time Lord. So nobody ever liked him because he never developed. He was just obnoxious (and obnoxiously dressed, instead of the black velvet he was supposed to wear or something).
JNT strikes again, I guess.
