Identifying someone as a criminal without charging them and thereby giving them an opportunity to clear their names has due process implications, largely because of the associated reputational damage.
DOJ policy therefore advises against naming unindicted co-conspirators:
Ordinarily, there is no need to name a person as an unindicted co-conspirator in an indictment in order to fulfill any legitimate prosecutorial interest or duty. For purposes of indictment itself, it is sufficient, for example, to allege that the defendant conspired with "another person or persons known." In any indictment where an allegation that the defendant conspired with "another person or persons known" is insufficient, some other generic reference should be used, such as "Employee 1" or "Company 2". The use of non-generic descriptors, like a person's actual initials, is usually an unnecessarily-specific description and should not be used.
The policy comes from a case where they did identify two unindicted (alleged) co-conspirators, and the Fifth Circuit ordered the expungement of the portions of the indictment that named them, agreeing with their claim that they had a right a due process right "to protect their reputations ... against the opprobrium resulting from being publicly and officially charged by an investigatory body of high dignity with having committed serious crimes." U.S. v. Briggs, 514 F.2d 794 (5th Cir. 1975).
At first blush, the description does seem to fall into the "unnecessarily specific" category, but note that the language doesn't actually say "Individual 1 became President in 2017," it says that Cohen's conduct occurred "in or about January 2017," and that Individual-1 had become President "at that point." So if the conduct occurred before noon on January 20, 2017, we'd be talking about President Obama.
That's obviously hyper-technical, but we are talking about the Department of Justice here.
Another possibility is that the success of the campaign/conspiracy may be necessary information for sentencing purposes. I'm not all that familiar with the Federal Sentencing Guidelines, but my understanding is that a sentence may be reduced if a defendant had withdrawn from a conspiracy in time to prevent its fulfillment. Because that did not happen here, Cohen would not be entitled to that reduction.