The ikemen rejected the gyaru. With clear self esteem issues, she immediately set her sights on the next guy (MC) even though she'd known him for years without ever being attracted to him. This was such a flaw in the narrative that in later chapters flashbacks, to earlier years at school, are inserted in a failed attempt to address it.
Like most high school boys, the MC is a pushover.
Their relationship won't last. Arguing over minor issues is fine at the start of a relationship. In a few years they'll be sick of each other.
The best girl was cast aside because readers preferred the gyaru option, with author recognising gyaru = more volumes.
For the record, I enjoy the series and Saori Oniishi is one of my top 5 VAs.