Adria at SearchEngineWatch speaks truth when she lists what are, in reality, just a fraction of the reasons a blog writer may not be interested in what a PR person is pitching.
It has, thankfully, been quite a while since I had to personally engage in any such activities in my role as an agency person. For a while there I was pretty good at it but somewhere along the road things shifted and those who were starting blogs started expecting more than what was being offered, mostly in the form of compensation for writing anything about the client I was pitching.
This corresponded, I think, with the rise of not just more pay-for-play tactics but also platforms like Twitter and Facebook along with more brands getting in the publishing game themselves. The dynamics changed drastically to put much more of the power in the hands of the blog owners themselves, which is a good thing in the long run but which has made this sort of social media outreach a much more difficult trick to turn successfully.