You may not realize how many there are total because most of them are regional. Things like Spectrum or Charter or Verizon or Cox or CenturyLink are all broadband carriers that were never available where I am, so I only heard about them through others online. Meanwhile, neither you nor this announcement mentioned my ISP.
As many of those are not part of this inquiry, presumably the FTC is not worried about the privacy practices of those ISPs at this time, and just targeting ones it has specific concerns about.