IF you are talking about Freeman's Mind...
You do realize that Morgan is often suited for the calm, wise old man personality? From what I gather, even in movies, like Now You See Me, his character never explodes into fits of rage until he found out he was framed at the end. Take another example like the Almighty movies, he's God.
You know, it's kinda creepy if you have Gordon shooting stuff up while speaking softly and be chill about everything. If you were to spend some time imagining Morgan taking up the character of Gordon, it's weird. It sounds more like a Let's Play. Here we have Ross to fill in the role of a psychopath (who arguably has a higher chance of surviving in a situation like Half-Life), which is perfect.
But if I read your question wrongly, were you referring to story/movie/documentary narration?
If so, go to Youtube, watch Ross's Game Dungeon, and also watch Through the Wormhole (narrated by Morgan), then you may see the difference.
It's about genre, really. LIke I said, Morgan's best suited for slower-paced stuff, but we often hear Ross like a gibbering genius, even when he reviews Eternam.