I don't know enough about the history of the discipline of political science to answer the direct question, but on the general question of why "disciplines" split off and fracture:

"Disciplines" and "fields" emerge historically and in the present day both as a matter of specialization and intent, but also for sociological reasons (e.g. the seizing of "turf"). The questions of a "political philosopher" and a "political scientist" may look similar to a layman but often these include wrinkles of difference that are readily apparent to a practitioner (history of science and philosophy of science superficially have a lot of overlap, but the practitioners of each can barely communicate with each other most of the time — our interests, methods, evidence, questions, genre of speaking, and so on are very different despite a few areas of overlap). Often new disciplines come out of the founding of new societies and journals, which are in part a way to say, "this way of doing things is best and different from that other way of doing things, and if you agree with me, publish in my journals and join my society." This is the sociological payoff, and it has material implications in terms of jobs, bosses, grants, and so on.

If you are interested in this, the basic "starter" article on the subject of disciplinary formation is Pierre Bourdieu's "The specificity of the scientific field and the social conditions of the progress of reason." As the title implies, it is not a breezy, beach-side read, but it is a serious sociology of science assessment of why disciplinary lines are drawn when they are, and how they are.