- Sherlock does not remember his past, Mycroft does.
- Sherlock does not know James Moriarty has met Eurus, Mycroft does.
- Sherlock does not know his sister and her condition, Mycroft does.
Mycroft knows what Eurus is capable of. He does not know that Eurus has the control of the island until they were trapped in her game. He seems unable to do/contribute to anything.
Now this is the man who always knows everything, even before Sherlock solves the problem. This is the man who heads a secret organization and has an intelligence equal to or exceeding Sherlock's.
So, why was Mycroft unable to solve the Final Problem?