Elisande largely made that decision on behalf of Suramar, and after the Sundering they thought the rest of the world was a wasteland, so they never went outside. When she got exiled, Thalyssra realised the rest of the world had continued on without them, and she wanted to connect with it to help her people. Over the course of the Suramar campaign we help Nightfallen who want to stop Elisande, having grown tired of her and the elite, and want to rejoin the world. They're assisted by two main elven factions; one who say "hey, we understand what you're going through with addiction, we'll help in any way we can", and one whose leader says "you betrayed us in the war, you filthy ingrates, I'll help you because we need to stop Gul'dan, but you're as worthless as those other elves who are helping you out of sympathy as much as strategy".
It doesn't matter that the Nightborne deserted the war, particularly since at the time Tyrande and the others were rebels: the Nightfallen acknowledge their error and largely want to make up for it, and Tyrande is just shutting them down and acting like it's a distasteful chore that she has to help them. It doesn't make the Nightfallen much indebted to her, especially when another group of elves is being much kinder and respectful toward them, claiming to have gone through a similar problem in recent years.
If you've got a criminal who sincerely regrets their past actions and wants to atone asking for help, and one of the people who feels obligated to help them complains about it and keeps reminding them how terrible they are, who do you think they are going to appreciate more? If someone wants to change, you may be angry or hate them, but at least they want to change; you shouldn't spit on them and deny them any more help than the bare minimum. It only makes them reconsider trying to make amends.