Couldn't be arsed
I know a couple of people who were unable to vote for unforeseen reasons, they being respectively a severe health crisis and an unexpected work call-out to another country. Not every failure to vote was due to apathy.
The point was often made that older people were both more likely to turn out and more likely to be for Leave. I might add that pensioners also have more time on their hands to actually go to the polling station on a work day. Why doesn't Britain do like other countries and field votes on a Sunday?
-A.