It likely has to do with pressure backpage is receiving to curb human trafficking. Although there are many independent woman on backpage there are also many who are being pimped, often underage...not good, but fact. Perhaps the powers that be feel forcing the social media link will make it more difficult for traffickers to use back page. But as previously mentioned, backpage is not enforcing the legitimacy of the social media accounts. If that is the case then forcing advertisers to post social media links becomes pretty much useless.