The explanation he's given is that the primary purpose of the website is to help prospective graduate students evaluate strength of CS PhD programs when deciding where to apply. For that purpose, it only makes sense to include people who can advise CS PhD students. This excludes not only research done in EE/ECE departments, but also research done in companies, or in departments without a PhD program (there are some strong ones at undergraduate-only colleges), since that's outside the scope of finding PhD programs.
That's fine as far as it goes, but the website has gotten popular as a general research ranking for a few of these subfields, beyond the original purpose of helping CS grad students choose programs to apply to. For some of those other purposes the original decisions may or may not make sense. The name of the website probably doesn't help either.
