Because Chasidish boys get married at around age 18 (usually between age 17 and 19) to Chasidish girls of the same age (18). So there are an even number of boys and girls in the shidduch market.
In contrast, among the Litvish the 23 year old boys marry the 20 year old girls. Since there is an age gap, with the average Choson being older than the average Kallah, there are less boys and more girls in shidduchim. So there will always be a certain percent of Litvish girls left over who can never marry.
The solution (on the communal level) is for Litvish boys to follow the Chazal of Shemone Esrei L’Chupa. By doing this, marrying younger, it will reduce (or eliminate) the age gap.
The solution on the individual level is for Litvish girls to get married as early as possible, so that they should not Chas V’Shalom be the one who is left over with no one to marry.