Because in short,that's really all this book is save the final Wham Episode
at the end where Snape kills Dumbledore
and Harry vows to find the Horcruxes.
While it does start off alright with the introduction of the affable [but not evil] Horace Slughorn, that plot goes out the window
quickly once Harry sees Malfoy at Borgin and Burkes,which is where the main plot is. Quickly it becomes Harry's near Foe Yay
obsession with Malfoy being a Death Eater.
On the side,Rowling gives us the past of Tom Riddle and revives some elements from Book 2 which covers the first half of the book that ISN'T Harry obsessing over Malfoy.And like Slughorn it too gets tossed over for a chick-flick subplot which should've been Character Development
for Ron and Hermione but truthfully wasn't,and it put Lavender Brown far too far into the spotlight. The main character meanwhile just suddenly falls in love with Ginny,who is dating Dean to drown her once Unrequited crush.Unfortunately Rowling forgot to actually write in any interactions between Harry and Ginny,until out of the blue,they kiss,...and start dating. And once they do start dating [in the last few pages] we are only told secondhand,and then they just break up. Not to mention that pitiful Operation Jealousy
thing going on with Mc Laggen
Gee considering Sirius Black died,and the threat is shown to have escalated as of the Prologue,you'd think this book would clear things up and not be so much Lighter And Softer
.But nay,it is a return to light-hearted Prisonerof Azkaban
Unfortunately all things wrong with this book,flood into the next one,making Order Of The Phoenix
truly the last good book of the series