Indeed, the simplest explanation is that they are hearing from voting constituents that porn and other objectionable content is too easy for kids to get online, and want to be seen as "doing something about it."

Most parents don't want their kids looking at porn. While there are steps they can take to prevent it, they require some technical knowledge and are generally easy to get around. The easy availability of this content is what has changed. You used to have to go to a seedy bookstore, "adult" movie theatre, or video rental business to get it, and they wouldn't let kids in. Also you had to pay for it, and most kids don't have any money.