What makes a didactically effective lecture?
Unless you put a lot of thought into your lectures, it's hard to adopt the mindset of a student who knows much less than you.  You may take certain concepts for granted, and you've long forgotten how you learned them, or what originally confused you.  At the time you learned them you were probably much more well-prepared than the average student, so common stumbling blocks for students were never issues for you.

This means there are a lot of professors who are good at giving talks (to people well-versed in their field), but poor at giving lectures (to people with little or no background).