They are differentiated when you evaluate them as an administrator/requester. Reporting, integrations and cloud storage handling narrow competition to the few pricier options available.

They are generally the most expensive, and people buy. The brand has value.

They have customers whose products/services call into Docusign APIs at dozens of locations. It's sticky.

if you (as a sales team) can get docusign recognized official in an enterprise, moving to another solution is going to be a difficult transition.