From "The Analyst":

Putin has decided to replace the defense minister Sergei Shoigu, Andrei Bilousov is the new Minister of Defence

Reference: Andrei Bilousov, aged 65, previously served as Minister of Economic Development of the Russian Federation from 2012 to 2013. From 2013 to 2020, he worked as an assistant to the president, and since January 2020, he has held the position of first deputy head of the government.

So the question is why? Incompetence, failure, boredom with hearing the same old stories and lies? Corruption gone too far?
Shoigu hasn’t gone too far, having been appointed to the role as Chairman of the Security Council, a presidential advisory body. Not that it does much advising. It’s often used as a place to put lame ducks out to die. They sit and make odd noises and gain praise - Medvedev is a prime example.
The most obvious thing is that the new defence minister has a background with a deeper economic outlook- and that’s something if the war is to continue at its current pace, that will be needed.
A grip or economics and the realities of supply and demand matter when the economics of the war are shredding economic realities.