It just goes back to considering who your target audience is in order to market your blog. There is no reason why niche blogs should lean on SEO/target ads, when they could more effectively just directly advertise in like minded niche placements and capture their desired base. If you have a website full of videos about car repair, buy ad space on vwvortex forums. You can't ever possibly know all the disparate secondary interests of your target audience (although FANGs are trying to model this), so stick with what you know, your niche.