Actually, "because I enjoy it and am good at it" isn't a bad place to start, but you can build that out more. As you studied this area more you realized that you really enjoyed it and were good at it, which led you to take more courses and ultimately to specialize. That shows that it's an ongoing interest, while "because I'd done 90% of it already" sounds more "opportunistic" -- you did this because it was there, but it might not be something you'll want to be doing in a year or two.
You don't say if you're an undergraduate or graduate student, but undergrads especially have very little to distinguish themselves in coursework -- you're all taking mostly the same courses along the same trajectory. Pursuing a specialization is good, but when we interview students what I and my peers (technical people, not HR people) are mostly looking for, after basic qualifications, is that passion and enthusiasm that tells us that you actually want to be doing this. That turns out to be a pretty good predictor for success.
While it's not exactly your situation, I've used a similar approach to explain a career shift. When asked about it I say something like: "I was a perfectly capable X, but as I spent more time doing Y I found that I was really good at it and really enjoyed it, so instead of being a capable X I set out to be an excellent Y."