This has to do with your PageRank value, aka link popularity aka page “worth”. Basically the pages aren’t “worth enough” to index.
Googlebot is a dumb piece of software – it goes out, finds pages to index (based on links it follows plus links it already knows about) and dumps the code from the pages into a database.
Other programs analyze the page content, images, code etc. The page’s PageRank (PR) value is calculated around this time as well.
PR is mostly derived from links pointing to the page. If there aren’t enough links, or the value of those links is low enough, then the page hasn’t earned a high enough value to be indexed.
In the case of your site, let’s say all your links point to your home page, and all your internal pages are only interlinking with each other. That means those pages only “earn” link pop (PageRank) from each other. The homepage will have a higher PR because of the external links. It then “shares” that PR with the other pages. So if your site has 10 other pages each page “earns” 1/10 of the PR value of the home page. That is then added the the value calculated and if it falls below a threshold (that only G knows) that page is considered to be of low value.
The way to increase the value of the page, then, is through building links. More links generally means higher PR (or link popularity, or worth, whatever term you want to use). And it is that higher PR value which helps ensure your pages are not only crawled, but indexed and showing in Google.
I have GREATLY over-simplified how crawling, indexing, and PageRank calculations work here, but I hope you get the point. And that is – your page has little value currently. Increase the value of the page to get it indexed.