Because it took 3 Super Kamehamehas from at least a FPSSJ(shit, prolly a SSJ2 since by this point in the series Goku "should have" aquired it), a SSJ2, and a SSJ, along with his own at least planet destroying attack sending him through the entire sun at FTL speeds, and he was not killed til the pressure exploded his heart, and since he died his powerlevel went to 0, which means his immense power which gave him his durability was gone.