Short answer: rasing interest rates has its own costs to the economy, and significant jumps carry much higher costs, so they only want to do it by the absolute mimimum they need to.
given the Economy is an almost incomprehensibly complex machine of feedback loops and confidence games, its basically impossible to accurately predict the amount they need to jump to ahead of time. Ergo, the make smaller, more mesured inceases, which give people time to adjust to each raise and can better see how much they need to go.
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