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)