What do reviewers mean by "broad readership" when rejecting paper?
Though I haven't seen this language used, if it was the editor saying "broad leadership" rather than a reviewer, I suspect what they mean is that they did not feel your paper was sufficiently revolutionary for their tier of journal. The editor would likely base this decision on lukewarm reviews, even if those reviews are not fully negative. 

Your options in this situation would be to 

A) publish in a lower-tier journal, to make sure your work takes precedence in the field even if it lacks some exposure, or 

B) better emphasize the novel aspects of your work in a future submission.

I also like the suggestion by @Ian_Fin that "leadership" may have been a typo from "readership" which I think would make much more sense, but my conclusions and interpretation are the same: this is the editor saying your work is not sufficiently groundbreaking for the tier of journal and either belongs in a more specialty journal or you would have needed to improve the writing to clarify the significance to the broader community.