The common theory is they did it to boosts Google plus. They wanted you to follow blogs interest on G+ rather than more directly via Reader/RSS.
> "While the product has a loyal following, over the years usage has declined."
[0] https://googleblog.blogspot.com/2013/03/a-second-spring-of-c...
While not exactly as strong as a conspiracy theory, I think they were well aware of what they were doing, and RSS was in the "best if absent" feature category.