Lenses may have different focus markings for infrared light because, basically, different wavelengths of light are (usually) brought into focus by a photographic lens at different points. This is also the reason we can experience chromatic aberration in photos (see below). By adjusting the lens's focus according to the infrared focus marking, then the infrared component of the light will be brought into focus at the correct point on the focal plane. There's more information about that in this existing question:
Why is infrared light's focus point different from that of visible light?

Returning to chromatic aberration, one way lens manufacturers counter this is by using apochromatic lens designs, which basically use special types of glass, e.g. fluorite. These lenses are better corrected to focus all wavelengths of light at the same point - including infrared light. Canon L lenses use fluorite lens elements. More reading at the Canon Camera Museum:
Fluorite lenses: Corrective capabilities beyond the limits of ordinary optical glass

Basically, the answer to the question is that Canon L lenses don't need infrared focus markings because there's already been an attempt to design/manufacture them in such a way as to focus infrared light at the same point as visible light in any case. Some L lenses may have infrared focus markings, which should be used when applicable - because not all lenses can be corrected perfectly.