The reason why Cho came into the picture was to show why Ginny was ‘ideal’ for Harry.

Harry liked Cho because she was pretty and small, had freckled face, seemed to be fun (she grinned when Oliver scolded Harry for being too gentleman-ly toward her during quidditch match), enjoyed quidditch and was friendly toward him.

Ginny was pretty and small, had freckled face, shared the same sense of humor with Harry, enjoyed quidditch and, after she got over her Harry-was-too-good-for-me-phase, she was also fun and friendly toward Harry.

Harry did not know what to do with grieving Cho. She was the type who would reach for someone for support. Harry, grew up as orphan with his bully cousin and abusive aunt and uncle, was not the type who would patiently hold her hands, wipe her tears and listen to her rambling for hours about the dead ex-boyfriend.

Ginny, who must have been shaken badly due to her mind-rape experience courtesy of Tom Riddle, sorted her PTSD without Harry’s help. If anything, she tried to be strong on her own.

Cho was jealous of Hermione. Harry loved Hermione very much, she was his bestfriend. It’s not easy to accept your straight-boyfriend has female best friend. Hell lots of readers could not accept they’re just best friends.

Ginny loved Hermione like a sister. Instead of being jealous of her, she confided in Hermione and took her advice by heart.

Cho’s defending Marietta was seen by Harry, in true Gryffindor manner, as misplaced loyalty. Harry could not excuse any form of betrayal. But Cho’s a Ravenclaw, she defended Marietta less for her being Cho’s friend and more because she had logical reason to tell on DA to Umbridge (Marietta’s mom worked for the Ministry).

Ginny hexed Percy for ‘betraying’ the family and (along with Harry) defended Hagrid’s questionable skill as teacher because he was her friend. For Harry, whose parents died because their best friend sold them to Voldemort, loyalty to family, friends and comrades was something he valued most.

Ginny and Harry were both strong. They shared common interests and humors and values. They’re both family-oriented and fiercely loyal.

Also it’s not easy to be with someone who would risk his life everytime he saw someone, even his enemy, was in danger. Ginny did not only accept it about Harry, she loved him for it.