Why does this happen?

I guess you mean why the trophoblast failed in modifying spiral arteries, right? If that's the case, unfortunately, the pathophysiology is not properly understood but we do know that this invasion would fail 40% of the time for people who have experienced pre-eclampsia in the previous pregnancy.

Stylistic suggestion: Think about using the commands <divclass=pull-right/left> </ div> (add a space between div and class, and remove the space between /div) for more aesthetically pleasing image placement and slightly increased readability.

Thanks. I would definitely use that in the markdown for my future article.