There are several reasons someone would advertise on your site but there are certain things that would make them come even more to advertise on your site.

One of the things i have noticed to get private advertisers to your site, your site must look nice. No one will advertise on a ugly cluttered site. So decluttering your site will increase the chances of somene advertising on your site.

Another factor for getting private advertisers on your site is to make the advertiser section stand out. Who wants to advertise and pay for their brand to get out if the ad cannot even be seen. So as I said, make that advertising place stand out and you will see more people flocking to the site to advertise.

Traffic. If you can prove to people that have a lot of traffic, that is probably the best and first thing they will look at. Traffic is what drives the money on the internet. You have no traffic you cant make any money. There is a rule. First traffic then money. Without traffic no one on the internet would make any money.

There are way more than just these ways to get advertisers to your site but by trying these you are already 5 steps further.