1. Your goal is to reach your flock. A blog allows you to do that in much the same way that a weekly email or bulletin insert can do, but it remains more permanent. Emails get deleted, bulletin inserts usually get trashed, but blogs don't go away.
2. Blogging is a good way for your flock to see what products/resources you'd recommend. Reading a review from you about a new book will mean a great deal. Just saying it in the bulletin or mentioning it on Sunday morning will be easily forgotten, but reading a review on it will help keep that resource fresh on your congregation's minds.
3. Your schedule: Are you speaking at a conference? Can you post the audio or video from that conference on your blog? If so, then it could be a real boon to your congregation to be able to hear what you say. Let's imagine that you're speaking at a leadership conference. Don't you think your members would like to know what you say? Post about it and keep them updated! Furthermore, with the global reach of social media and blogging, don't you want them to invite their friends to your event?
4. Your writing. Many pastors and church leaders today write books or resources on the side. Even I have a few short ebooks that talk about issues near and dear to me. Pastors can extend their sphere of influence past Sunday morning by including writing, both through blogging itself and in promoting books.
5. Building community: The most important reason to keep blogging, or to start blogging if you aren't already, is to stay engaged during the week. Any given pastor is going to be pulled away in a number of directions throughout the week. However, a blog, with at least weekly updates, can give you and your members a way to keep in touch. Just make sure you reply to comments! Otherwise it might backfire and you look even more distant.
As you can see, plenty of reasons exist to keep blogging. Our congregations need to hear from us when it is appropriate to keep connected and keep expanding spiritually.
Are there reasons you blog as a pastor that I didn't mention? Please do so in the comments!