The reasons why I may not always post comments on your blog:
A) You have word verification on. Sometimes I have trouble with that.
B) If you are not on blogger, I may not want to go through all the steps of posting my email and all the other hoops you have to go through.
C) The post was too long to read.
D) The post did not interest me.
E) I don't visit your blog.
F) I was pressed for time.
G) I didn't feel like it.
H) You never comment on my blog.
Seriously, if you use comment moderation, you don't really need to use word verification. Word verification is just annoying.