Owen and Beru have to die so that Luke is an orphan Hero. Also, because it's why Luke is instantly a die hard rebel with a cause. Also, because someone who matters has to die horribly so that we understand the Empire is evil. This is the US, there are lots of people who prefer Empire and find great satisfaction in seeing its enemies slaughtered, so this had to be dramatized so our sympathies fell right.
As to why it is right and proper that Luke be so unfeeling, well there was a severely compressed time frame,. Pausing at all, much less pausing to indulge a downer moment for personal loss, wasn't really in the cards.
Nonetheless when all is said and done, the writing for the first movie simply wasn't that good. Any perceived deterioration for the prequel trilogy is imaginary.