Class action lawsuits and MDLs are similar in that they both combine large groups of people who are essentially filing the same lawsuit against the same defendant.
However in MDLs, cases are combined for the pretrial portion of litigation and then transferred back to the court where they were filed. With a class action lawsuit, cases remain combined throughout the entire legal process.
Plaintiffs in an MDL share the same complaint but the extent of their injuries can vary significantly, which is why they are unable to move forward as a class action.