There are a few reasons why a horde of game clones appears shortly after a game becomes a hit, and the reasons all revolve around intellectual property.
First, the game developer probably does not have any patent protection because they probably have not invented someone new and non-obvious.
Second, the game developer probably hasn't acquired any trademark rights to the name.
Third, you cannot copyright game mechanics (17 USC section 102(b)). Copyright only protects an expression of an idea. Thus, everyone can create their own interpretation of a hit game.
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