Why would a "poorly written" paper get a reject and resubmit decision rather than major revision?
A "major revision" decision generally implies that, if the reviewers' concerns are addressed, the paper will probably be published. (See this related question, What does a &quot;major revision&quot; mean?)

Sometimes when a paper is poorly written, it's hard for reviewers to judge its technical merits. (Because the presentation is so poor as to make it difficult to understand.) In these cases, a "revise and resubmit" decision often makes a lot of sense. It wasn't possible for the reviewers to properly evaluate the paper, so it's hard to say with any confidence one way or the other whether the paper will be publishable if the presentation is improved. The obvious course of action is for the authors to improve the presentation and resubmit the paper for a proper review.

Based on your description, it sounds like that may be what happened in your case, as one reviewer wasn't able to comment on anything besides presentation.