I'd say mainly for those reasons:
1) It was quite an embarrassment for all those young people who did not vote and will suffer now
2) the actual results of a Brexit are known now while the bus-advertisement that was supposed to be the benefits of a Brexit are gone and have not been replaced by anything
Also all those news about businesses reorienting towards Europe and away from UK, Ireland, people applying for EU countries citizenship and so on.
The real results became real while the people who are still for a Brexit have shown their ugly faces.
I'm pretty optimistic about the results of a second referendum. That's why the Brexiteers will never allow it.