Why can parrots imitate the human language and other mammals can't?

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Mammals have relatively bigger brains than birds that will allow them to learn more than fowls can. But a question keeps bogging me every time I visit the local zoo here in Manila. I usually hear the Parrots speak my language but the usual house or work dog cannot imitate even the simplest words. Though I have encountered dogs and cats who sing along with a certain tune, but they usually make funny noises that can not be understood by the human mind. What's so special with parrots and other talking fowls? If you have any idea, just post your comments and responses and they will be very much appreciated.