Why would it be a poor choice to commute using a mountain bike?

I only bought a mountain bike for commuting purposes and because this particular bike has a lightwieght aluminium frame, 29" rims, 27 gears and a 3x9 drivetrain that will allow ke to ride at high speeds. But why would a mountain bike make a poor choice for commuting. I only bought a mountain bike for... show more I only bought a mountain bike for commuting purposes and because this particular bike has a lightwieght aluminium frame, 29" rims, 27 gears and a 3x9 drivetrain that will allow ke to ride at high speeds. But why would a mountain bike make a poor choice for commuting.

I only bought a mountain bike for commuting and its gear selection for high speed riding.
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