Planets do not twinkle because
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they emit light of a constant intensity
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their distance from the earth does not change with time
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they are very far away from the earth resulting in decrease in intensity of light
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they are nearer to earth and hence we receive a greater amount of light and, therefore minor variations in the intensity are not noticeable
Planets don't twinkle because they're much closer to Earth than stars, so their light appears as extended sources rather than point sources. Atmospheric turbulence causes the point-like light from distant stars to scintillate (twinkle), but the larger apparent size and greater brightness of planets make them less affected by this atmospheric disturbance. This is why planets appear steady in the night sky while stars seem to twinkle.