This is an easy one.
Yoda gave us the answer in "The Empire Strikes Back". When Luke was meditating and saw into the future he asked Yoda if his friends would die, to that Yoda replied, "Difficult to see. Always in motion is the future." Meaning it's ever-changing, affected by many unforeseen factors.
Palpatine had become so arrogant that he believed his clairvoyance was unshakable. What the Emperor didn't see was the fact that Luke was trained by Master Yoda (whom he presumed dead) not just Obi-Wan, that Luke's faith in his friends was warranted because his friends were very powerful because of their strong bonds, that Leia was actually Luke's sister which made Luke even more powerful through that bonding love and lastly and most important that Luke's Father did indeed still have good in him as Luke had 'felt'. The last, Palpatine found out the hard way when Anakin's love for his long lost wife and his and Padme's living children out weighed his loyalty to the Sith. Palpatine had believed he was so powerful he had single-handedly change the Jedi prophecy, that the chosen one would bring balance to the Force... he mis-read, and in the end Anakin did bring balance to the Force.
