Question

why can the sun's heat not reach the earth by conduction and convection ?

Answer 1

it's because the distance between sun and earth that it is space so the suns heat can only reach Earth through radiation

Answer 2

if u remember ,then you have been taught that heat can transfer in conduction when there is two isolated body is in contact whereas in convection fluid is required.
but between earth and sun there is vast vaccum that's why sun heat reaches earth in the form of radiation...hope u understand