Bernie has a practically fixed number of supporters. When elections roll around, he’s quick to pick them up and make a respectable showing in polls - but doesn’t garner any more support than that. The number is enough to challenge competitors, but not enough to win outright.
If Bernie is rising/leading in polls, I suspect it’s because those not supporting are not answering the poll - out of dismay for the alternatives. Bernie’s true believers are solidly behind him. Those who aren’t have a choice of ... Biden? Warren? Buttedgeedge? ugh. Maybe non-Bernie Dems are just quietly sitting this one out.
In a Democrat world, Iraq just got free stuff from Iran.