Because of the less quality of those links. If you build high quality back links then they will be considered for long time by search engines.

Because; may be you're back links are quality less.Quality (dofollow + high PR)

Building High PR backlinks and edu backlinks are the best ways to get ranked

Welcome to the dilemma that all spammers face.

You are creating web spam on other websites. The moderators discover your spam and delete it. Those websites that aren't moderating spam are devalued by search engines. All your efforts are washed away like a sandcastle when the tide comes in.

All you can do is rebuild your sandcastle like backlinks after each spam fighting tide washes them away. It seems you have embraced a very Sisyphean marketing strategy.

A different path would be that of the meritorious link builder. This type of link builder builds backlinks on a rock solid foundation of useful content, using relevant anchortext as the stonework for a marketing strategy that multiplies your efforts, like a stone castle built on solid bedrock that lasts for generations.

It has something to do with the quality of the links. Search engines are very sensitive when ti comes to the quality of the links specially Google.