3. If Hank Does Figure It Out, He Could Just Keep It Himself
Think about it, if you'd spent a year or so of your life investigating the biggest meth distributor in ABQ, and then found out it was your brother in law, would you go straight to the D.E.A with that information. Nope. Questions would be asked, for sure. Why did it take you so long? Why did you never suspect him before? How much was he paying you to keep the case going? It'll do more harm than good to go after Walt after all he's done. He could just keep quiet, keep chasing leads, and arrest someone else in Walt's place, maintaining the glory, avoiding the questions and embarrassment.