In eukaryote cells cargos are often transported cooperatively by kinesin motors and nonmotor microtubule-associated proteins (MAPs). The prior in vitro experimental data showed that the velocity of the cargo transported by kinesin motors and Ndc80 (a member of MAP) proteins of truncated coiled-coil stalks decreases sensitively with the increase of the ratio of Ndc80 to motor number. However, the underlying mechanism of Ndc80 affecting sensitively the cooperative cargo transport by kinesin motors is unclear. To understand the mechanism, here we study numerically the cooperative cargo transport by kinesin motors and Ndc80 proteins. Our results showed that for the case of the motors and Ndc80 proteins with truncated short stalks, as used in the experiments, the calculated results reproduce quantitatively the prior experimental data. The mechanism of the cargo velocity decreasing sensitively with the ratio of Ndc80 to motor number is revealed. By contrast, for the case of the motors and Ndc80 proteins with full-length long stalks, the velocity of the cargo decreases slowly with the increase in the ratio of Ndc80 to kinesin number. Our results thus give an explanation of why the kinesin motors working in the cell have long stalks.