For me I see the reason why the first PoP sold better than the later 2 is because:
1) PoP had some ground-breaking graphics for its time.
2) It was the first PoP in a long time, & looked very promising with bright ideas & better 3d implementation.
3) News on the development of the next 2 PoP's showed that the developers were focusing alot on the combat aspect, whereas I believe people would've been happier with more puzzles, more challenging puzzles.
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I think they thought of my 2nd point & thought that the longer they waited to release a new sequel, the better the sales. That's a correlation, but it's not proof that this method works. I agree that people will be driven away for that. Now, if they said they're delaying it so that FOR ONCE they can take more time on polishing it up & finishing it, THAT would actually make me want to play it more. If they were lying, then their loss.