Why are the dispersion forces in CS2 stronger than the dipole-dipole forces in COS?
Although individual dispersion forces are weak, they are cumulative, and increase with molar mass.  As a general rule, boiling point increases with molar mass. 

Polar molecules will have higher boiling points when compared to molecules with similar molar masses.  For example, ethanol($\ce{CH3CH2OH}$) has a higher boiling point than dimethyl ether ($\ce{CH3OCH3}$).

$\ce{CS2}$ is ~16 g/mol heavier than COS.