The biggest problem for the IAF was the Egyptian air defense. If the Israelis committed a preemptive strike which would require the majority of their air force, they would have lost a huge chunk of it. If they expected 67 part two, the remaining units would be utterly demoralized.
Report: Palestinian, 18, killed in clashes with IDF in West Bank (Haaretz)
from the article: Newly released documents: Division and disarray on eve of Yom Kippur War