Because PageRank is just a Google measurement that counts the number of backlinks for particular page. So each page will have its own PR. Here you might see one - maybe more - of your older posts has a higher PR than your home page (your domain).
It's not the case for this blog;
- Home page (After moved to custom domain 3 months ago) - PR 3.
- Previous post (Old Blogger dashboard will be removed) - PR 2.
- Top landing page (Smooth scroll back to top in fixed bar with jQuery) - PR 2.
Back to June last year, this blog experienced PR 4 after 5 months on the web. It was constant during the next 2 PR updates until I changed the domain.
Why need to improve PageRank
Having a better PageRank means Google considers your particular page (your article) more in SERP (Search engine result pages). This is good for niche blogs. Most web users use search engines like Google to find information. They will type in related keywords to see the most relevant results. And definitely they will not type your domain there. Here is where the PageRank shows its importance to help your posts be among the top results. So you need more efforts in building QUALITY backlinks (Usually come from high PR sites). Be a guest post over there. Write a very fresh content or a new finding that people may use as reference. You can get massive traffic from social media sharing strategies but they are all nothing for the purposes of PageRank.
In my opinion it's really difficult to point what actually did the trick when seeing our blogs go up and down in the SERP. Sometimes, a new competitor with considerably fewer backlinks and less contents gets ahead of us. As far as I concern, the best trick is don't ever trick Google to improve our blog's standing. Stay away from getting penalized and keep yourself motivated.
For those who were not satisfied with the results, you don't need to worry as the major PageRank update will come in next few months (as early this July maybe).