Because he's not preparing a legal defence, he's simply delaying his trial.
Given the preponderance of evidence against him, I'll credit him with attempting to take the most effective course of action available to him at the time, whether it be hiding from arrest or delaying his trial, but his course of action is now limited to facing accountability for his actions, so he's screwed anyway you slice it and his supporters are just going to have to spend the rest of their days bemoaning yet another great injustice perpetrated against Serbs.
How do you know he’s not preparing a legal defense, are you a mind reader? What purpose would be served by him delaying the trial? I don’t see how it benefits him. He’s in prison as we speak — it’s not like he’s getting away with anything. If he’s convicted he winds-up exactly where he is now — in prison. What does it matter to him if he’s in prison before the trial or after the trial? Prison is prison.
If he’s guilty and he gets convicted by means of a sham trial, where he wasn’t able to present the defense he wanted to present, all that does is undermine the credibility of the verdict. What credibility does a conviction have if his supporters can rightly say that his fair trial rights were violated? If you think he’s guilty you should be the first one defending his right to defend himself and have a fair trial, because if he doesn’t have that the trial won’t prove anything.