Why do radial tires have low rolling resistance?
Radial tiers have a lower rolling resistance than previous types
of tire
Resistance is energy, in tires resistance creates heat.
Radial tires construction and the materials they are made of
create less heat and roll easier.
And if you need a more detailed explanation you need to look at
Bias Ply tires (called Cross Ply in some countries) and Radial
tires to see why it makes a difference