Same thing has happened to me countless times. Most of my playthroughs start out optimistic but stagnate and end at castelia or, at t latest the next city.
I think it's partly because the region isn't too exciting. It's too "realistic" -- the towns are so urban that it makes you feel like you're walking through the world that you played to escape from. It's depressing and lackluster.
Another reason is that all of the unique and interesting pokemon don't start to show up until after castelia, except for one or two. By the third badge, you've already been playing for the few hours so the novelty has worn off by then. I wish they could have put pokemon like pawniard, joltik, golett, rufflet, shelmet, and so on earlier. Replay value goes to shit with these games when it's always patrat and purrloin for the first gym. Compare the selection of pokemon available to you pre-gym 1 in Crystal or Emerald.
Lastly, the handheld story drags things out so slowly and never really makes you feel like you're out on your own making your way. Things move slowly when your friends pop up at every new town.
