I read Twilight. Why was this so popular? The theme of unrequited love? Longing? Passion? Works every time. Didn't read any of the others. One escape book in a row was enough, but gotta give her credit. She did what a lot of us would love to do.
As to "why", even Stephen King chimed in his celebrity $0.02 about how nudge-nudge metaphorical the story was for pubescent teens first feeling their hormones flowing and their blood pulses starting to race...
As for "Why do they read the REST of them?", I hearken back to a possibly apocryphal post on the Internet (very likely a hoax, but if so, an incisive one), about an angry fangirl who wrote an open letter about the "Wolf Man" remake that was still in theaters, saying they got the werewolf lore "all wrong" compared to the handsome, brooding CW-series instant-changers of "New Moon".
...There is no loyalty so fierce as that which teenagers feel they have discovered themselves.
(Call me judgmental for giving up on the Percy Jackson series after one big-studio movie, but writing styles aside, think the OP could watch Twilight's first movie and pretty well get the mindset for it in a nutshell--
Should be checking out either around the baseball game, or the unique revelation that sunlight actually makes sensitive, caring vampires "sparkle like diamonds", rather than be burned to ashes as creatures of the night....The girls want the lure of the "good" bad-boys, but they don't want to be seen tying themselves to the "bad" bad-boys.)