The election had higher voter turnout than any national election in the UK in decades, and most of all polls showed the election was going to be very close. Blaming voter complacency does not seem logical.
So, in the spirit of democracy, where the desires of all those who /can/ vote (not just those who /do/ vote) are respected, it is probably interesting to examine why voters do and do not find the motivation to vote. Hypothetically, if we had mind-reading devices, and motivation was not a prerequisite for voting, which side of the vote would have benefited?
Asymmetrical motivation is the hypothesis that in referenda where Action/No-Action are the choices, there will be more voter motivation for Action rather than No-Action. This very reasonably applies to Brexit.