Why? Because relative humidity measures the amount of moisture in the air compared to the current temperature of the air and what it can hold at that point. While few point merely measures the temperature of the air at which it reaches dew point and dew falls. . The basic conditions for a thunderstorm are moisture, unstable air and lift. It would be easier to read off moisture level in the air since relative humidity at 90 to 100 means it will rain more certainly than it will with dew point which may only mean that dew will fall.
