There will be a million and one reason why this might be, but I always start with any e-commerce site by looking at the content. For this example, I looked at the following page:
http://www.grandcrayon.ca/hlr26411-hilroy-coil-scrapbook-26411.html
There isn't a lot of content on there, but if you look at the description "Studio scrapbook features durable manila paper pages and oversized coil binding for easy viewing without tearing." in Google, there are another 23 sites using this as well.
Another example page has this description "Jumbo straight straws are designed for use with cups that hold up to 32 oz. Straws are translucent and individually wrapped to keep them sanitary." and in Google, this shows another 19 sites using it.
So, as a first take away, look at writing some unique content, although I appreciate this could be a big job.
Looking at your site in Open Site Explorer, you are lacking on some good links, so I would be looking to expand on this as well. If you don't have much in terms of site power, internal pages will suffer.
As I was on your site, I noticed it is quite slow, and after testing this, it isn't great (although I have seen a lot worse). Google doesn't like slow sites and it is bad for usability too. When testing in Page Speed Insights, it does give you some suggestions.
Do your internal pages not feature in Google anywhere?
-Andy