Because it would be incredibly repetitive. Sure, it'd be sad, but nothing Telltale could do would top Season 1, Episode 5's ending, Clem having to kill or abandon the only real guardian and closest friend she had in the apocalypse.
What would they do? Have Sarah shoot Clementine before she turns? Have Luke have to put her out of her misery? Have a walker come out of nowhere and devour her at the last second?
I'm not saying the ending can't be sad, but Clementine dying isn't the answer. There's just nothing Telltale could or would do that wouldn't feel like a replica of S1E5's ending or would be effective.