Bobby hadn't proven himself at that level yet (it was just 1958, Bobby wasn't a monster until the mid 60s early 70s). Petrosian was a former world champion. For players like you and me it comes up in OTB tournament play (online it doesn't matter, anyone can offer a draw). If you're lower rated, it's bad etiquette to offer a draw. Because the stronger player "should" win, and knows more ways to test the position to see if you really can draw it, you should wait for them to offer first. When they're out of ways to try for a win, and after you've proven it's a draw, they'll offer it.
When the weaker player offers a draw first it's kind of like saying, "I'm as good as you and I know it." Not a big deal, and I can't imagine it would earn you any enemies, but it is a bit rude. If you decline the draw from the stronger player, he wont offer again until you offer. Because you've made it clear you won't accept a draw, etiquette says the next one to offer a draw will be you and so on.
In the 3rd game, Petrosian vs Fischer, Portoroz 1958 (King's Indian Defense)
Fischer mentions at the end that he offered the draw, not realizing it was bad etiquette. It was Petrosian's place to extend the offer.
I am relatively new to the rules so can someone explain this to me. Is it because white should be the one who asks for draw or is it something else ? I thought either player could request a draw.