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Because Pokemon forgot that exploration needs to feel like you're actually out there exploring something. Starting with BW, the regions are more urban than rural, more civilized than wild. You're also no longer alone, there's just about always company. Everything is clean and safe now.
Additionally, BW is when Gamefreak started to push the story as more than just a background that you don't need to give a shit about. That's where we first got cutscenes and deeeeeep dialogue.