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
Sorry, I was gone for a while, so I didn't get the chance to respond yet. Maybe a little late, but I still wanted to, so here I go:
First of all, I want to make clear that what I wrote isn't what I think happened. It was pure speculation if we followed the conspiracy theory path. I think what I wrote in my edit is much closer to what I think actually happened.
I might have been a bit quick to say news outlets wouldn't go in depth about it, I just hadn't heard that much about it yet in my country at the point of writing it. Thinking back on that, it was kind of a baseless conclusion of mine anyway, and I regret making that statement.
However, I do think part of my statement still holds up in the sense that if we do go that path and he would be "killed" immediately after imprisonment, the impact and outlash would've been even bigger.