Lists only allow for major versions. Major versions signal that the information or data is "production-ready," but in lists, that is not always the case. For example, the default "Announcements" list that comes with SharePoint Online could easily involve the drafting of an announcement that isn't ready for prime-time, because it hasn't been reviewed and/or approved by another author. With minor versioning, SharePoint authors could keep the draft version from being seen and inadvertently consumed by site visitors until was ready for publishing, at which point it could be set as a major version.