Believe it or not, the "Let $X \in F$..." style is typically easier and more straightforward to write for mathematicians and theoretical computer scientists and

Believe it or not, the "Let $X \in F$..." style is typically easier and more straightforward to write for mathematicians and theoretical computer scientists and closer to how they have the solution in their head (or on scraps of paper) anyway. The problem is, unless the result is super important, that style will get your paper rejected as it doesn't engage the reviewers who are not already familiar with the problem. So authors go instead for the conversational tone and try to find a good story, which is more often what actually shields that the work is quite simple or less important.