As the turnout implies, the anti-brexit camp had it harder to mobilize, since the status que seemed quite safe and unlikely to be challenged by many, especially, being already in a pro EU-region. So a lot of people did not come out to vote in favor since they seemed their vote not neccessary. Asymmetric mobilisation and de-mobilization is a thing.
https://en.wikipedia.org/wiki/United_Kingdom_European_Union_...