SEO - search engine optimization and Google rankings are again an ever changing landscape. I mentioned that I mostly do design and site creation/maintainence - that's where I specialize. I kid you not, the Internet is much like medicine where there are SEO specialists, social media experts, hosting and server specialists, coding, and on and on. All of it changes all the time.
Search engine rankings are based on all sorts of things. There are a couple things working against you here. The first is mobile friendly. Google puts mobile friendly sites ahead of others. How often the main page changes - Even if other pages change it can look like the site is not changing which again lowers the rank. Perhaps most important is many sites these days are based in Wordpress - which has all sorts of SEO plugins that can be used. These greatly help with rankings. If this were a Wordpress, mobile friendly site, just that alone would increase rankings.
All of that said, we are also very increasingly a social media world. People visit and use Facebook, Twitter, LinkedIn, etc far more than visiting individual websites. There are a dozen or more Facebook groups that deal with the same content as here. Someone posts a question at 9 am and by afternoon there could be over 100 responses. All of these things compete with website communities.
We here are old - really old in electronic terms. Wordpress and Facebook were infants when the sub forum was started. It's nearly like three generations since then or more. Think of all the places that have come and gone,major we are still chugging along.
I have not even mentioned the sites using Adwords and other methods to try to boost traffic. The thing is, many are selling something or in some way generating income while you are doing this mostly to be a nice guy! It's much easier to throw a couple grand at something when the income that will result is ten grand. Again, as someone who does this on a pretty regular basis, you are doing great - far above the average content provider or business. Be proud of what you've accomplished
All that said if you really want to press on, looking for and converting over to a CMS based forum like Wordpress and incorporating a social media presence would do the most to attracted more people. If not, I see nothing wrong with being the jewel of an out of the way corner bar where many know each other and everyone loves to visit.