After having watched 20 episodes with Andrew’s voice as Dean, then to hear Jensen now, I have to say, Andrew really did do a good job. I didn’t like his voice at first, but it grew on me. He had big shoes to fill and I think he succeeded.

As to why Jensen didn’t do all 22 episodes - I have no idea. I’ve never seen it posted anywhere his reasoning for only doing two. You could be right, it could’ve been the same time he was working on Tron.