Why would you include a tutorial if you're not going to put the effort into making it good? Imagine how engaging your favourite films would be if they had no sound — just subtitles (hint, it's not as immersive/fun since your focus is divided).
Well considering part of Sins inspiration is games like Master of Orion and other old school 4X games, they didn't even have tutorials. You had to read the manual to learn to play those games. And that is basically what the tutorials in Sins are, a much more limited but guided session of reading the manual.
Don't get me wrong, the tutorials have a lot of problems, mainly that they are no where near detailed or inclusive enough to get new players up to speed, meaning they'll spend a couple of games getting killed until they mess with stuff on their own and figure it out. However, in my opinion the lack of voice acting is very very far down the list.
