1. The author of the blog understands that they have a readership and wishes to maintain a convivial exchange with said readership.
2. The author of the blog assumes the readership will have noticed or be disillusioned by the inactivity on the site.
3. The author of the blog presumes that the blog medium inspires a sort of fickle reading relationship where new content is the primary reason for readership in the first place.
And then there's number 4, which I believe is where I fall:
4. The author of the blog wishes to assuage their own guilty feelings over prolonged inactivity by assuming an audience, thereby having a rationale for apology to someone apart from just themselves.