Because it's a thief game? The point of doing sequels or reboots is drawing in the existing audience who enjoyed the older games or those who heard good things about the older games and use them as a base. That's like saying "okay, we want to make another Tomb Raider game, but it's been a while since the last one and we can't get one of the original voice actresses anymore so we'll just create the same game with another main character model and throw away any chances to get the old fanbase on board".
Doesn't sound like a smart strategy to me especially if the new game still has enough familiar elements to be considered part of the series.
He's important but not critical. It's a bit like the James Bond series...fans grow up with a certain actor portraying the role and when a new actor is hired to take the part they kick a fuss and cry that that guy is not Bond...then the whole cycle continues and when the torch is passed again, the same thing happens with others. That's how nostalgica works.
The Bond analogy doesn't work because voices last much longer than a person's appearance/physique do. Peter Cullen has been doing the voice of Optimus Prime for nearly every Transformers tv show, movie, and game for 30 years now. Prime wouldn't be the same without his voice.
Another good example is Frank Welker and a lot of the voices he does. He has been doing Fred from Scooby Doo since 1969. He *is* the voice of Fred.
It's not nostalgia, it's an appreciation of voicework. I'm sorry you don't value it and willingly accept any ersatz voice.
I appreciate voice work, but I also see the practical difference between having done three games over the course of 6 years and having done hundreds of cartoons over the course of several decades. The average gamer who spent his teens giving the thief series one runthrough before moving on probably wouldn't even be able to tell whether it was a different actor or Russell who had aged 15 years.
And I guess keeping an open mind and judging voice actors on their own performance makes me unappreciative of voice work. Hmmm, that doesn't sound right.