This is because the heart needs enough time to pump blood out to the main arteries.
The action potential of the heart (ventricles) is elongated by the opening of the voltage gated Calcium channels causing an influx of positively charged Calcium ions to the inside of the muscle cells and therefor maintaining the depolarized (excited) state of the cell membrane for a longer period of time. This is phase 2 of the action potential which is also called the Plateu Phase.
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