My impression was they took the Rebels more seriously. So sent the Emperor’s blood hound after them with a small fleet and a fancy command ship. Palpatine is playing with fire knowing that if Vader finds his child there will be sparks. But he also knows it will be a beacon to finding the rest of the Rebels that fled Yavin.
I sort of get the impression that the Emperor lost faith in guys like Tarkin after the first Death Star fiasco, and gave Vader more autonomy.
I attribute it to the character and film being a global cultural phenomenon lasting years, so Lucas decided to change his significance and history to increase profits. No story beyond '77 suddenly became hire someone to write some sequels, this thing is huge.