Question

why cannot heat passed by means of conduction or convection in vaccum

Answer 1

because there is no matter in vaccume but for heat transfer require a medium to transfer

Answer 2

Because in conduction and convection matter is required fot transporting heat.

I.e. In conduction heat transport in solids so there is solid molecule but in convection molecules of liquid and gas is needed.

But in vaccum there is no molecule for this purpose here only radiation can work.

Hope it helps you a lot....