Well, that's in character, as he's not really a team player, and the current crisis is a very personal one to him so he probably feels he needs to handle it alone. He'll probably get grudgingly pushed into accepting the Legends' help more as the season goes on. And a lot of the characters right now are off doing their own things.
That's about contracts, not story content. If a new cast member is added to an ongoing series, their credit normally goes at the end, since the other actors' relative credit placement is set by their contracts and it wouldn't be fair to demote one. But if the new cast member is a bigger role or gets paid more than some of the others, then they get a special notation before their name to compensate for the low position. I believe the practice was created for "Special Guest Star Jonathan Harris" when he was added to Lost in Space after the pilot and they didn't want to bill him lower than the three child actors. But it's become standard ever since. In Matt Ryan's case, he formerly played this character as the star of his own series, so it stands to reason that he'd be entitled to a high-ranking credit.
On the other hand, the Arrowverse Wiki lists him under "Recurring Cast," but I'm not sure if that's accurate.