Because the rulers of that country dont want to end dictatorship.
If democracy comes, they will be removed.
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A political system like democracy, socialist, communist depends upon many things.
China living in peace since 3000 years under one rule and now they have communist type of govt, where as in Indian case there were different kingdoms. Russian after industrial revolution exercised there right in workers type government through august revolution. But later they gradually modified into democratic system. It totally depends upon area specific people mindset and there struggle against existing sytem to reform it in better way. 
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