If you prefer, the answer is that he diverges from what ought to be the message to be dispersed among possible voters, too much.
Therefore, yes, sometimes he needs to go to the background while others are criticized - common political approach to a campaign.
But I find it a little funny that you say that about Trump, because it's actually Harris (not Trump) who they have been hiding from the media, such is her shame on her terrible record - and Harris and Walz have been mostly just criticizing and name-calling to the Trump side.