Because Ben 's death was the origin of spider-man. Gwen's is not only the origin of the green goblin becoming a true arch enemy but also the origin of peters adulthood.
You wouldn't have the spider-man we know and love today for the last 40 years without either of those tragedies.
That's over-selling if ever I've seen it.
Gwen's death wasn't the origin(?) of the Goblin becoming Spidey's true arch enemy. That happened through all the battles where Spidey could never defeat the Goblin and was climaxed when the Goblin became the first villain to discover Peter's identity.
And Gwen's death was the origin of Peter's adulthood? So all of those years of his risking his life unselfishly was adolescence? Peter became an adult when he realized with great power comes great responsibility. You can even say this was capped off in the iconic scene when he lifted the impossibly heavy machinery off of himself in ASM #33 to save Aunt May. His choices made in battling the Green Goblin were the culmination of those years of experience.
After Gwen's death, there was nothing. He learned nothing from her death. He wasn't changed in anyway, other than moving on to a relationship with Mary Jane. And if anything, post-Gwen he became more childish.
He proposes to MJ, even though they clearly weren't ready for marriage.
He involved himself with Black Cat, a criminal.
He unites with the alien symbiote without knowing what it was.
He lets Venom roam free on several occasions, and even re-unites Brock with the symbiote simply because he can't handle seeing someone die.
He reveals his identity to the world, knowing the danger of it.
He surrenders his marriage to Mephisto because again, he can't handle seeing someone die, even though acceptance of death is the ultimate example of maturity.
Gwen's death is nothing more than a dramatic moment in Spidey's history. Not an essential one.