I think Matt liked his time at Iowa State and for many years it did not make that much sense to leave.
He probably had decent leverage in negotiating with Penn State (as they completely botched the hiring process), it gave him an opportunity to move closer to family, double his salary, and have the NIL landscape be easier to navigate.
He probably saw Heacock nearing retirement and thought it might be a good opportunity to try a fresh start. If he fails, Matt will still probably have a nice payout.