1 of 2

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!