Initially, Davis was planning on using the techniques from his new essence gathering cultivation manual, Incursive Barren Lightning Manual, ending this battle in a few moves, but he didn't think that he could summon his manifestation out. He did have an idea of trying it in the future, but the thought of using his manifestation in this battle never crossed his mind.
Furthermore, it looks like Halifan's Earthen Palm Manifestation was not a technique like he initially guessed, but it was a manifestation. No wonder it appeared larger but intrinsically weaker than the golden palm from Elder Towerfall. The power of the manifestation directly equated to the cultivator's prowess.