Breyer was a prominent copyright law scholar before he became a judge. Between that, and the fact that he hasn't gotten many opportunities to write majority opinions in his 27 years on the court, and the fact that he's likely about to retire in a few months, giving him the opinion was the obvious and collegial thing for Roberts to do.

I'm biased, but I don't think the technical elements of this case were challenging to an intelligent layman. I think most people smart enough to become judges can understand the concept of an interface that is independent from an implementation, which is all this case really required, along with quantitative estimates of the amount of code involved. So I doubt that he needed clerks to understand any of it.