Because that position has oversight of the Mueller investigation, which Sessions recused himself from. Trump has previously expressed disappointment in Sessions because he believes the AGs role is to protect him. The acting AG has previously stated he would limit the special counsels budget to the point it would be completely ineffective, and that he views the presidents finances as outside the purview of the special counsel. This isn't about whether or not president have the power to decide who is AG (of course they do), it's about Trump installing a toady to head the DOJ who will protect him from his crimes.