Need Some SEO Advise

Hey guys

I run an ecom store and have been doing my own SEO.

For one of my lucrative keywords, there's a subdomain that's ranking no1 while I'm at no3 for a few months now.

My site:
DR: 52
RD: 34

Competitor
DR: 2
RD:11

I optimise my on-page once per quarter so I don't think it's due to this and my site speed is 2x better than theirs.

The only reason I can think of is that their whole site is dedicated to that one keyword while mine is just a product page.

I'm still quite stumped as to why I'm not been able to outrank them.

Any suggestions?