I started blogging about five years ago, more or less in the manner you describe. Wrote an excited post or two or three right away, then maybe another in a week or two, and then... slowly stopped. Then a year later picked it up again, and then stopped. I tried a variety of blog tools. I first had a blog at MIT, then when they killed that service, I tried several of the hip new services like Tumblr, Typepad, and Poste…
Part of the problem is that many people see blogging as a teaching tool alone. More often than not, it's a conversation starter too, and the blogger ends up learning a great deal of new information in the process.
And even if you wish to be only a teacher with your blog, there is also a bit of Dunning–Kruger effect at play here. You think, what's the point of me explaining X? Everyone knows about X, probably better than me. Others wrote about X.
In truth, you'll find that more often than not plenty of people don't know about X. The way you'd describe and explain X may actually lead people to acquire a new prospective on the subject.