The hardest thing to achieve in publishing is a recognizable brand. There are so many authors out there that the average reader has time to give a shit about. To most readers, books are like potato chips; you go with the brand you like. It's why new writers clamor all over themselves to get a blurb from a recognizable author.
This seems pretty true to me. Most people if they find an author they like, they'll stick with it. There are a few authors I prefer above others: John Irving for starters, Lawrence Block for another. And most people have their favorites too.
Series, or even the authors, are like movie franchises: every publisher wants to have a big one just like every movie studio wants a franchise like, say, the Marvel superhero universe. Why else is DC/WB trying so hard to get their universe off the ground? Why do they keep turning crappy YA dystopian series into movies? Because if they get the next big hit it means billions in movies and merchandise, plus inevitable reboots, prequels, spinoffs, etc. It's the proverbial cash cow.
Unfortunately, franchises like Harry Potter or authors like Stephen King are extremely rare. Which as that quote above says is why agents don't want to let those things go. It's why they get so desperate as to do shit like publishing the same book from a different point of view or publishing the same story with gender swapped characters or exploiting an old woman on her deathbed to publish a first draft.
It's why I've been doing what I've been doing for 27 months now. I'm not anywhere near Harry Potter or Stephen King territory, but it's better than I was doing, so I keep with it. As they say in Bull Durham: Never fuck with a winning streak.