Why was "Case Yellow" more successful than "Sea Lion"?
"Case Yellow" was the code name given to the initial German push through the Ardennes and into France. Sea Lion was the name given to the invasion of England that never actually occurred. Case Yellow, of course, succeeded where Sea Lion was never even truly attempted. The major reason for this was the German inability to destroy British sea and air forces.
Case Yellow was successful because the German army was much more prepared and had much better tactics than the French and British. German mechanization and use of combined arms was beyond anything the Allies had conceived of.
Sea Lion, relied on Germany domination of the English Channel and the skies above it. This required the destruction or distraction of the Royal Navy (much stronger than Germany's navy) and of the RAF. Neither of these things was done successfully. Because the British continued to have significant sea and air power, the invasion was deemed impossible and was called off.