Because it is a choice. Valby is improving rapidly, while Tuohy is not. You know why? Because her coach don't want her to improve rapidly. Which is the right thing to do.
You say Tuohy is at her peak? What made you arrive to that conclusion?
Tuohy's coach said at the conference that her role has been holding her back, and that we haven't seen her best yet. At one interview the coach said they want their athletes to run the fastest on their Senior year, as it should.
Have you seen Tuohy run extremely fast times in cross country? No. What about Valby? Every. Single. Time.
Tuohy and Henes knows what they are doing, winning titles and not overdoing it. Pacing for the final year where it matters most.
If Tuohy was coached by Solinsky, the NCAA would get smoked. Solinsky lets her athlete decide. Tuohy has to beg her coach to run certain meets.