Most likely it is your DA (domain authority) It is currently only 18 and if your on page ranks that high for the keyword its just the juice/power your website domain has in the SERPs. Try some content building that is industry/keyword related and hopefully it gets shared to relevant sites that have a higher DA/PA than you so you can build some juice from those links. You could always look to purchase some links but that world of white hat/black hat SEO can get a little grey.
Hope this helps!