My guess/speculation/opinion is that it is related to the common pedagogical premise that the primary purpose of writing is to present a convincing argument. And passive voice is viewed as "weaker" in the context of debate and argument by conventional wisdom. For what it's worth, I think the emphasis on writing to argue and convince is a significant contributing factor to the behaviors people exhibit once they discover text boxes on the internet.

An alternative approach to writing is as a tool to examine one's own thoughts...and perhaps explain them to others. That is, writing can be a tool by which the writer comes to understand the subject rather than a means of teaching it to others. This is a bit more at odds with the idea that education is about filling up the empty bucket of a student's mind.