Because this new information is but the latest drop in Baines history.
Lorthemar has a background of egotistical selfishness and zealous protection regarding his people, rest be damned. People find those qualities somewhat positive.
They take said approach and react in kind.
Negotiating with the Alliance becomes something that he is doing for the benefit of those under him, and regardless of how someone personally feels about it, the reasoning he used in conjunction with his personality, makes said actions even understandable. Positive.
Now, we take Baine.
The one that excused the killing of his people, that went to his enemies looking for help when Voljin and the rest were just there for him to see, the one that argues for peace when it’s not sensible doing such, and that has consistently shown reticence fighting for his people when doing so implies going against the Alliance. Regardless of the pile of affronts they kept leaving at his door.
Of course people will see it in a different and less positive light even if we somehow decided that the circumstances were more similar than they actually are.
Lorthemar jumping ship is seen as simply another way to save his people. Baine doing likewise is seen as the usual cow favouring the factions enemies over his own. Again.
Portrayal and context of both characters matter.
It’s a similar reason as to why this ongoing revolution is seen in a worse light than the one Voljin carried out.
Ps: Also Cairne set the bar very high.