I had a thread on exactly this topic. I think its a lack of high enough quality links and too many mid/low quality links. Pouring more and more and more cheap PBN links and social shares is not the answer based on my studies. The only way up is ultra-high quality authority links, which is also ultra expensive. Which goes right along with what I've been trying to tell everybody here for years: Google isn't trying to kill blackhat SEO, they just want to make it more and more and more expensive to operate successfully until only a few elite can afford to manipulate the rankings. Then the rest are forced to PPC, and make Google rich. SEO is the free test of crack to their PPC junkies, you can cheaply rank to a certain level cheaply/easily... but no further. Then you NEED more traffic, and PPC is their alternative.
In any medium to hard niche you'll find page 1 almost impossible unless you can either spend GOBS of time or money. Build it, buy it, or bribe it.