There is a lack of explanation in this article. Schumer doesn’t “assign” people to committees like people assume here. Unless other long-tenured committee Senators, Bernie Sanders or Debbie Stabenow in this case, are willing to give up their ranking spots on other committees (Budgets for Bernie, Agriculture for Stabenow), that pretty much leaves Manchin as ranking member. The Committee on Committees is not going to boot a ranking incumbent member absent a contest for the spot, and it would likely go to a secret vote within the caucus to decide anyway.
Instead of yelling at Schumer, progressives should be pressuring Sanders and/or Stabenow. They are the ones that need to make a move. The problem is though, both probably prefer to be in line to chair those other committees than Energy.