I am not sure that what is the best way to show bbpress forum as main page in your wordpress site but you can try this approach:
suppose if you want to show forum archive page as main page, follow these steps:
1. Create a new page with shortcode [bbp-forum-index] and publish, for this follow these steps:
a. Add a new page from ‘Add New’ option from your wordpress site admin panel.
b. Enter a page title and add the [bbp-forum-index] in page body.
c. Click on the publish button.
(see this how to use shortcodes)
2. open Settings –> Reading in your admin panel.
Check a ‘A static page (select below) ‘ option, and select the new added page in ‘front page :‘ dropdown (see below).
and click on ‘Save Changes’