The UN is a meat grinder and not an automatic stepping stone to a higher "domestic" position (there are two reasons right there).
Secondly, I think she was spreading her wings at State.
She went from being spokesperson for State to being acting Under Secretary of State for Public Diplomacy and Public Affairs, the fourth ranking position at State.
Also, she became part of Pompeo's inner circle.
The Presidential Press Secretary is brutal and would be an odd lateral kind of advance for Heather, but it would make her a household name.
From the Press Secretary job, she could conceivably go anywhere.
“she became part of Pompeo’s inner circle.”
If so, and if Pompeo trusts her, that’s a great thing.