I would say that predicting the chances of an asteroid hitting Earth based on looking at a crystal ball is a pretty bad prediction.

I agree with your general point. Whether a better prediction could've been made based on available data is really the key question here, and this is exactly what allows us to say that crystal ball predictions are bad.

In the context of predicting election results, I think it is fair to assume that, in principle, there should be enough available data (or an ability to collect such data) to make more accurate predictions. This also seems to be the assumption of all major polling agencies, and was also the assumption in investigating the results of Brexit polls. This is precisely where it differs from dice rolling. It therefore makes sense to assume that the predictions were inaccurate due to methodological reasons, as opposed to pollsters having no practical way of accessing the relevant data.