If tor bridge relays aren't available to public and hide you from the government/isp... Etc then why there are smaller number of bridge relays than normal ones?
The Tor Project recently released a blog post talking about it. While the post doesn't say why the number is so small, it does talk about how to set them up. I believe though that the reason is simply because people don't know what they are or what they do.
My theory is that many users of Tor don't realize that it is a tool for anonymity and privacy and instead they just confuse it with the ridiculous "dark web" meme. A few will really dig in. Fewer still will set up a relay and barely more than a handful (relatively speaking) will ever set up a bridge.
How do we fix this? First, set up your own if you can! Blog about it, tweet about it, let people know what bridges are and how they can help the community, especially those in countries with heavy censorship.