Why Wal - mart wasn't be able to enter the city market?

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Wal - mart supercenter is everywhere it seems. But you can not find it in the big city such as New York, or California. That is the one of biggest curiosity to me. Some people said labor union in New York really against the entering of Wal - Mart, and they claimed there are already plenty of stores are good for locals to shop at. So, what is the real reason for that?