Honest question from someone who has never played Sonic Adventure 2 before. Why is Sonic unable to save Shadow? Like I know the part where Shadow loses his Super Form, but couldn’t Sonic… carry him?

In the final fight's action dialogue, Sonic says "Shadow, continuous use of your super form will cause you to dissappear"

and all the other characters on the ARK radio in to tell both Shadow and Sonic that if their rings deplete then they'll lose their super form, so they need to switch out with eachother so one can re-collect their super form while the other fights in their place.

It seems that Sonic's super-form was also ending at that same moment, so if he had stayed behind to catch Shadow, they would have both fallen, he probably only had seconds left. Additionally, in the Twitter takeovers (if you care to count that toward canon) Shadow stated-

"If I hadn't waved him away, Sonic would have fallen with me. But his determination in trying... I'll never forget that."

So we can assume that Sonic only had seconds left, and that Shadow was being purposefully uncooperative in order to protect Sonic. Also, nobody else had ever gone into their super-form besides Sonic up to that point- the vague wording of "disappearing" makes me think that Sonic wasn't even sure what would happen if he didn't return to the ARK.

Also, if you let the fight play long enough to hear more action dialogue, Sonic will repeatedly ask Shadow to return to the ARK, and repeatedly warns him his super form is running out, and will show concern and ask him if he's okay. It looks like he saw that Shadow was slipping, but couldn't get him to cooperate.

ALSO also, Sonic ends up returning to the ARK holding Shadow's ring, which means Shadow either slipped out of his grip, or Shadow purposefully left behind his ring to give Sonic enough energy to return to the ARK (cause as we said before, the rules of that fight was that you had to collect rings to power your super form)

Heres the video containing all the action dialogue in the final fight