1.) The main reason Gerry Conway chose Gwen Stacy to be the casualty in the story was not to introduce more turmoil to Peter Parker's life but so that he could promote Mary Jane Watson as the female lead of the series and flesh out her character some more. Primarily because he thought Mary Jane was the most interesting female character in comics at the time as she better reflected the changing attitudes towards women in the 60s/70s. "Women in Refrigerators" instances are hated upon because they are accused of using supporting female character's tragedies as mere plot device to further the character arcs of only male characters. It wasn't the case here. Mary Jane has a small participation in the story, but her very presence and implications of her behavior in ASM #122 separates DoGS from the rest of the fridging pack. A fan who wrote in to CBR during their "50 Greatest Spider-Man Stories" listing last year described this aspect of "Death of Gwen Stacy" incredibly well. You can read her comments on the following page, in the section for "Death of Gwen Stacy"-
2.) The second reason is because the story is just so damn good. The tragedy is almost Shakespearean in impact and the build-up and pacing is immaculate. Other WiR stories are just exploitative for the sake of being provocative. "Death of Gwen Stacy" is probably the greatest story ever to be told in just two issues. It truly deserves all the accolades it receives.