1. Black proves faster
2. Blacks majority is vulnerable
When one side has more pawns on a particular side of the board than their opponent they are said to have a pawn majority. When both sides have majorities, such as in position 1 (Marshall - Capablanca, 1909), the game can resemble a race to achieve a passed pawn: the faster majority wins.
In position 1, black's already mobile queenside majority is much more dangerous than white's kingside majority owing to the positioning of black's heavy pieces. In addition, the two kings being on the kingside favours black, as, in the event of exchanges and an endgame, white's king is a long way from being able to stop Blacks resulting queenside passed pawn.
However, every pawn advance leaves behind it potential weaknesses. Whether in terms of squares or amongst the pawns themselves.
In position 2 (Kramnik - Lutz, 1994), white won by compelling black's majority to advance. After 32... a6, leaving the b6 pawn weak, there followed 33. Rd6 Rc6 34. Nd4 b5, and white was driving black's queenside majority to its doom by targeting whichever pawn was left weak by the majority's forward movement. White eventually won by creating and exploiting a second weakness on the kingside.
See: 'Weakness' and 'Principle of two weaknesses'.