Site ranks good for all keywords except the important ones

[TadiBrothers]

Hey guys

I am the marketing manager for https://www.tadibrothers.com/, i started a year ago when the website was ranked very poorly on most keywords, I got most of our nitch keywords rank well to fist page except the most important ones.

backup camera 
backup camera system
wireless backup camera system
rear view camera

Our backlinks campaign is doing well 15 backlinks on average every month.
on-site optimization is also good.
Our Domain authority is better than all our competitors.

Can anyone Please help me to understand why they do not rank well?

Hope to hear from someone soon