He had the Treasury printing U.S. notes, not Federal Reserve notes that are laundered through the Federal Reserve system. That's the biggest reason I can think of. It always comes down to money because money = power.
JFK tried unsuccessfully several times to assassinate Castro. Castro promised Oswald a lot of money to kill JFK.
Johnson had Ruby clean up the mess, Democrat-style, so he could have his war with Vietnam and not be distracted with a war in Cuba.