Why was brown chosen to represent the ground side of an attitude indicator?
The answer to the specific question of "why PFD's (glass cockpits) use this standard" is because their old style gauge cockpits used it before them. The glass cockpits of today are made to be similar to the older gauges so as to avoid confusion in their operation when moving from one type of aircraft to the other. This has been shown to be an issue in the past in the difference between American and Russian attitude indicators as such instrument operation strives to be at least somewhat uniform. 

Now as for the more general question "why are attitude indicators Blue/Brown?" there are a few reasons. First off some types of color blindness cause issues with the blue/green mix making it a combination to avoid (some info on color blind flying). Second, and perhaps as important is that blue and brown sit basically on opposite sides of the color wheel making them very contrasty while blue and green sit right next to each other on the wheel. This ensures accurate viewing in most lighting situations. 

There are also other attitude indicators like those commonly used in military aircraft which have had different colors 


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The oldest examples are completely void of any color (you can see why they introduced color) 


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For what its worth some modern flight displays now include terrain data and are capable of showing many other colors to outline it as can be seen here on this Garmin unit 


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