Because after 2013 SEO is hard, Google rolled the panda, penguin, and hummingbird update in that year. Show me your analytics, I will show you the date and update name which affected you, I think hummingbird affected this site.
give me your webmaster and older analytics, so that I can all the things, also your website does not have well links structure, images alt tags, and content formation, meta is fine. also, the website needs good quality links.
I have analyzed that your content is not informative, it is like promotional content, you should mention information and unique solution in a proper way that you offer.
I want to know following details to answer your question
1. when and how many times you change meta tags.
2. when did you change the design and content of this website.
3. did you change links also or URL structure?
4. webmaster and analytics details
5. old analytics data date wise.
Your biggest mistake is focusing on a single keyword, dear your website has many pages so you should focus more keywords.
and seriously website is not good looking. at many some pages, you have many links that is also an issue.
Proposed Milestones
$5000 USD - To answer you the question and up ranking for this website