Ellison has never won statewide why would he get picked over someone who has?
People care too much about "overtures towards the Bernie wing". If Keith Ellison is the senate candidate than their doing to find dirt about him, not even mentioning the statements he already made in the past that will surely made the race pretty close in 2020.
Besides i doubt the Bernie wing is as active as people believe it to be and even care about the Minnesota senate seat in the long one and giving them a Senate seat that would become vulnerable because of Keith Ellison's past statements would be dangerous.