My guess - DocuSign has a hard to penetrate moat: It's known and accepted in courts. Everything else doesn't really matter.
Imagine you're a legal department. You have to choose between DocuSign, which you know the court will accept, or a competitor. DocuSign costs 10x as much as the competitor. But that's nothing compared to the cost of litigation, or worse, the cost of losing litigation. So you will likely choose DocuSign anyways.
Imagine you're a legal department. You have to choose between DocuSign, which you know the court will accept, or a competitor. DocuSign costs 10x as much as the competitor. But that's nothing compared to the cost of litigation, or worse, the cost of losing litigation. So you will likely choose DocuSign anyways.