I get this question a lot, nearly every interview, maybe because I have a science background. I think I hurt myself in interviews answering this question because I originally was saying stuff along the lines of: "the focus was too narrow and I thought law would give me more exposure to other areas" (more embellished, but you get the general idea). I might also have said in a few interviews that I just lost interest in that type of work ("bored").
Anyway, I went to our CSO and asked for some tips to improve my answer and they told me to answer more like "I wanted to work somewhere that would showcase more of my writing skills and since science and law are similar in application, my bg would be useful in a legal setting."
That sounded shallow and insincere to me, so I came up with this response: "It seems like jobs as scientific researchers plateau out pretty quickly, so I always kind of knew I wanted to do more than lab work my whole life, I just didn't know what. I was initially attracted to law, specifically patent work, because I knew it would be a good application for my background." And then I talk about how as I started to learn more about the law, more areas became interesting to me.
Any feedback before my next interviews would be great.