Why does the James web Telescope rely so heavily on infrared?
The design emphasizes the near to mid-infrared for several reasons:

high-redshift (very early and distant) objects have their visible emissions shifted into the infrared, and therefore their light can be observed today only via infrared astronomy;
infrared light passes more easily through dust clouds than visible light;
colder objects such as debris disks and planets emit most strongly in the infrared;
these infrared bands are difficult to study from the ground or by existing space telescopes such as Hubble.