DigitalOcean became popular through a simple pitch - it was the only cloud provider that **guaranteed** that your virtual server would be hosted on an server with a solid state drive (SSD). This, along with it's simplicity (no IAMs or security policies to configure) and very low price point ($5/month for entry grade server) made it very attractive to many.
The "SSD guarantee" was a big deal at the time, back when SSDs were still an *expensive upgrade*, and made DigitalOcean virtual servers perform faster then other typical cloud providers, especially for disk heavy applications.