It's all about integrations.
If you already have Office 365, it's natural to use OneDrive, since it's included with the subscription. Same goes if you already use SharePoint; OneDrive and SharePoint share a backend now (so you can enhance the filesystem with more metadata).
If you already have Apple devices, iCloud Drive is the easiest to get into.
If you already have a lot of friends with Dropbox, you'll probably get Dropbox.
Box has its niche in the enterprise segment, so it mainly competes with OneDrive. It's like Slack vs. Teams.