Relevant to note:

• Katara's original VA Mae Whitman did not want to return. She last voiced Katara in 2011.
• Aang's original VA Zach Tyler Eisen retired from acting after the end of the original series, and has not publicly commented on anything Avatar-related. Every Aang appearance in Korra and games since 2011 has been voiced by others.
• Toph's original VA Michaela Murphy has said that she "would love to come back" but is "respecting Avatar Studios choices". She has not voiced Toph since 2011, every appearance in Korra and the games was voiced by others.
• Sokka's original VA Jack De Sena has not commented publicly. He also has not voiced any version of Sokka since 2011.

So, to me it sounds like only Michaela Murphy and possibly Dante Basco would have wanted to return to the roles anyways? Coincidentally both of them were involved with Korra, voicing General Iroh and a young Suyin Beifong.

My cynical Translation: we're not going to be paying actors anymore.