Multicellular life seems to be one of the biggest stumbling blocks. Life appeared very soon after it was possible, but a pretty huge chunk of the time since was limited to single cells.
Keep in mind how many single cell organisms you can have in a single drop, how many drops are in the ocean, and how long a billion years is. Even a small competitive advantage turns into exponential growth for the community of cells. The noise of random mutations is going to eventually lead to a cooperation between cells that's going to cause a feedback loop.
Cell -> multi cellular seems easy and obvious compared to no cells -> cells.