**You can check the trustworthiness 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 that 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 their **Facebook** account at 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 fewer elements. Ask: Why do users use 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 a **because** statement can increase the overall rate.

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

Good luck!