The fundamental idea behind using a suppression list is to exclude audience members with specific data attributes. For example, those that might have previously opened a mailing in a resend campaign, folks that have already clicked a particular link, or maybe you want to exclude folks that live in a particular area, or did not open a previous mailing. The reason why we've prioritized the exclusion of segments is because they are dynamic and designed to cut through the broader groups you might be using, thus the idea is to be able to be more specific without the rigamarole of creating reverse segments.
That being said, there are plans for further iterations on this and I'll be sure to put in a feature request to make sure your voice is heard on. I'd love to hear your specific use-case for wanting to exclude groups if you'd like to share!