Why the crew wasn't more aggressive with electric trim when MCAS first kicked in is one of the great mysteries of this accident. As to why they didn't use more electric trim in the final moments, see this article for some informed speculation at Bjorn's Corner/Leeham News in the article "Bjorn's Corner: ET302 crash report, the first analysis." (I would have normally included a link, but apparently the forum rules don't allow me to as a newbie member)