Exit polls in General Elections do not happen everywhere. The polls take place in a few selected marginal constituencies. Those most likely to change one way or another, are seen as a barometer for the rest of the country.
In this referendum, voting is going across party lines. So there are no marginals, or if there are, no one knows where they are. After Scotland and Wales which will be safe Remain voters, the rest is very mixed, so it would be impossible to do an accurate national poll.