I suspect there is a factor in polling you haven’t considered. A lot of people simply haven’t bothered or had time to look at any of the candidates in an election coming up but don’t want to admit to a pollster that they are ignorant. So who are they most likely to say they support? The people with the most media attention. In fact you could say that there is probably, at least with polling anyway, a certain proportion of people who want to “back a winner”. Again these people will go for the highest polling candidate. Unless a candidate can present a strong separate brand (eg Trump, or SNP in Scotland) then as soon as they start trailing in the polls they will lose a lot of support.