After Elena went away for some personal work, Tomoe was chosen to be the leader of the group which would be going to the Ancient Alchemist's Workshop. At first Tomoe wanted to reject this idea as even though she was confident in her strength, she was not confident at all when it came to leading a team.
But Elena asked Tomoe to do this as it was going to be a good experience for her. That was the only reason why Tomoe became the leader here. At first she wanted Alisia, Turin, Grace and Anna to not go. But Alisia did not want to miss this chance, so she started to request Tomoe again and again to allow her.
Later on, Tomoe got convinced by Alisia's insistence and the strength she demonstrated. It turned out that since coming back to the vampire continent, Alisia had been working very hard and she was very close to teaching the A rank. So Alisia was more than capable enough to hold her own in this expedition.