Why? Because it craps all over the comic books. One of the most beloved stories and characters in Spidey history. It ends up being a great and tragic revenge story.
Norman is the father of Peter's best friend, making the battles a lot more intense. It makes Peter's meetings with un-Goblin Osborn a bit intense.
Osborn is a sick **** who ends up murdering Peter's true love and ends up dying himself (in a great iconic way I might add).
Harry, Peter's best friend becomes the Green Goblin and it makes their battles a lot more tragic, since they were once friends and now at eachother's throats.
Harry would never, I repeat NEVER harm Gwen like that. He even told MJ flat out he wouldn't ever hurt anyone innocent. Peter/Spider-Man was his only vendetta.
Now we have in this film, Harry coming in, and according to viewers, having barely any time with Peter on screen as friends, becoming a villain (a stupid looking one at that, who doesn't resemble any Green Goblin I have ever seen) for the stupid reason of "I'm sick I need your blood lol oh look there's Gwen OH HEY YOU MUST BE PETER", MURDERING Gwen Stacy and being a smug little prick about it at the end. That is not Harry Osborn. Harry robbed Norman of everything major that the comics built him up as. Starting the Goblin legacy, killing Peter's love.