Seven years ago, Republican donors failed to coalesce around an alternative to Donald Trump, allowing a candidate who was under 30% nationally as late as February 2016 to walk away with the nomination as a divided field could never top his celebrity power.
This cycle, Trump is in a stronger position, but, particularly in the early states, he is still below a majority of Republican support. Gov. Ron DeSantis (R-FL) is best positioned to beat Trump, especially in Iowa. The problem for other candidates, especially Nikki Haley, is that DeSantis voters prefer Trump as their second choice, while Haley voters support DeSantis.
This means that if Haley would exit the race, DeSantis would gain ground on Trump as more Haley supporters would join DeSantis, but if DeSantis were to exit, Trump would benefit since more DeSantis supporters would move to Trump. Haley simply has no credible path to the nomination.