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Just a reminder - It was Gen 4 that made gen 5 sell less. Gen 4 (D/P to be precise) is the one that people hated for being slow, have not enough new pokemon, shallow characters and being very tedious.
Platinium fixed all of these things. Thats why its saved Gen 4. But Many people LEFT the franchise by then and they did not supported gen 5.
ALso Gen 5 had really good Map with lots of backtracking and exploration. (B/W2 had even more).