1- (Tinfoil hats please) This is a state owned attack, which is a retaliation from US Government to ruin Twitter's credibility and introduce social media regulations.
2- The hackers are gray hat hackers, who know that reporting this vulnerability will not make them any money and they want to get what they think they deserve, so they make it public and get some good amount of cash.
3- The hackers had realized they had a massive vulnerability in their hands by accident and did not know what to do with it.
I find second and third option plausible, which also reminds me of the npm hack, where a very, very popular library was compromised and installed on a huge amount of developer machines, but only thing they did was to try to get hold of some bitcoin accounts.
I do not condone any type of crime but in both cases, it feels like a huge opportunity was missed by both hackers.