So Bastilla gave Revans mask to Meetra Surik so she could give it back to Revan to bring back his memories after she freed him from Dromund Kaas. Why didn't Bastilla hand the mask over to Revan before he left in the first place? The whole adventure of finding the Sith Empire etc. would then be superfluous.
She didn't think that he would need it. She thought that Darth Malak was the last Sith she thought Revan was going up against the Mandalorians who he had already beaten into submission once already. She didn't think that he would need any extra power or help.