Because the other probabilities add up to 0.7, or 70%.
The probability of the first player winning is also the probability of the second player losing and vice versa.
ie. First player has a 0.5 chance of winning and 0.2 chance of losing, whilst second player has a 0.2 chance of winning and 0.5 chance of losing.
Since chess only has 2 players, and if one wins the other loses, the only other possibility is a tie.