**You can check the trustworthy-ness of your product and service.** If users are choosing e-mail registration, there can be a trust issue. It may not be the company itself but the contextual information is important for users. 

As in the context, a user can enter a job application site with **LinkedIn** account and a person may not want to use **facebook** account in a **dating site** at first sight.

I think that it is not related to design of the form, the form design and message should be direct and with less element. Ask Why do users uses e-mail during sign up rather than social media accounts?

Social accounts do not have the same value in user's perception, try to explore them with users. Any **Reasoning** which has **because** statement can increase the overall rate. 

-  Join with (this social account) because ....... (this message should create a good reasoning)

Good luck!