This is pure speculation from me but I’d say because enough time has passed to make it relatively irrelevant. The war and Russia in general are back to being minor news, so even though a lot of news networks will probably mention his death, I doubt they’ll go very in depth about it.

Why not later? Why take the risk? Only a matter of time until enough of his allies gathered up and create turmoil to pressure his release. Or for some foreign influence to get involved when they realize his worth in taking on the Russian regime.

Edit: Other option is that they just planned to let him rot and didn’t care too much about allies. Probably got mistreated and suffered from exhaustion, dehydration and malnourishment. Only a matter of time until the body gives up