Because I'm getting slower in my old age.
Truly I thought it would go faster, but anyone who has ever written a book knows how much work goes into it. The changes are more than a few minor manuscript updates, too--there's a lot of things that have changed since Spring 2.0, which the previous edition targeted. So bringing the book up to date was quite involved.
And to top things off, I've been rather busy with my new day job for the past year--if you're curious as to what I've been up to, visit
http://www.springframework.org/spring-social.