To be fair, it is only one of many compounds that are effective against specific traits of cancerous tumors. In this case, it is effective against tumors which

To be fair, it is only one of many compounds that are effective against specific traits of cancerous tumors. In this case, it is effective against tumors which are poorly vascularized. In most cases, one of the predominant features of tumor progression is the increase in vascularizing growth factors from cancer cells. Tumors which fail to start pumping out growth factors that encourage blood vessel growth often kill off the entire tumor due to lack of oxygen. Also, the apoptosis cascade can be hijacked in many locations. Even if you get the mitochondria to start releasing Cytochrome C (the main component that triggers the cascade), cancer cells can turn this pathway off anywhere downstream of CytC, such as turning off production of one of the various caspases. So, again, it comes down to cancer biology being incredibly complicated.