The reasons can be multiple and most are perfectly logical. If you are not updating your home page on a daily basis - why should a spider waste bandwidth checking on a daily basis?
Most SE spiders are looking for new content on pages that are frequently changed or seeking links to new pages that have been created. So, as is usual for static home pages, crawl frequency can be reduced as there is likely to be nothing new (or of critical importance) to add to the SE database.
There's a side benefit with the "static" or "evergreen" pages in reduction of bot activity, but the SITE itself should show new content/pages on a routine basis to keep the bots coming back. The home page should show change...
How long has the site been up, and what time period has elapsed before the bots slowed down?