What was the impetus behind making Steve Austin "better than before"?

Did this choice of words reflect on social norms about those with
physical disabilities at the time?

"Before" he didn't have disabilities or injuries. "Before" he was an astronaut - in perfect health.

"Better than he was before" is followed by "Better. Stronger. Faster" [Queue theme music]

All it had to do with was the fact that he's got superhuman power due to the bionics, and now they have a man worthy of a superhero TV show. The only thing it had to do with is why he stands out - where he got his abilities. It lays out the premise for the entire series. there wouldn't be much of a series if there weren't a bionic man to do all the cool stuff he gets to do.

(Although how he used the bionic arm without tearing it off his body is still beyond me.)

This was the era if Dirty Harry, of Captain Kirk getting into fistfights, Bruce Lee, and Chuck Norris. The Lone Ranger was still cool, as were Superman and even Spider-Man, who got a cheesy series in that era.

We valued gritty tough heroes (as opposed to the tough anti-heroes we often see today, or the cartoonishly tough guys played by Stallone and Ahnold in the mid-late eighties).

Steve Austin was the type of man that we looked up to in that day. The ideals absolutely played a part. But not from a "disabilities are weak" standpoint. The bionics gave a good guy strength and made him interesting.