Well, there are so many answers to your question, here are some reasons why Google does not index your back links.
1. Your back-links are not worthy of indexing
2. your content is very thin and plagiarized
3. you are creating back-links from sites with no-index tag
I will give you the powerful and effective hacks that will help you get your back-links indexed fast.
Build Relevance Links on Authority Sites – How to find relevant authority websites for backlink purposes
Some of the best tools for this regard are:
Have Patience – Wait on Google to Crawl and Index Your Backlinks
Ping Your URLs Via Backlink Indexer Tools