1- its not popular. If its popular, it will either sell out for more fans and sacrifice the story, or the elitists will hate on it and change their tastes so they can like things that are less well known. Better known as the Star Trek effect.
2- Something happens in the series that did something better/new that makes it noticeable/rememberable.
3- A character like Cougar from S.C.R.Y.E.D dies. This happens in every great anime.