I think maybe there isn't just that as many activity on your site. If your content remains the same, or if user interaction is low, then with the same content available, the spiders would just be rescheduled for another crawl sometime later, but not too soon.
If you are able to consistently put content, and increase the user interaction, such as letting the users browse more pages on your site, or have communication directly on your page, and with all these fresh content, the spiders would definitely be attracted to these.
So with the above addressed, you can start to build quality links to your site, those that aren't spammy but good ones, and with better authority from those links, your site will enjoy better crawl rate after that.