The reasons are Geometric penumbra and Source Strength

Geometric penumbra

• It is due to the finite size of the source

The width of the geometric penumbra (Pd) at any depth (d) from the surface of the patient is given by

• s is the source size
• SSD is Source to Surface Distance 
• SDD is Source to Diaphragm Distance

From the above image its clear that as source size increases the Geometric Penumbra increases
In case of cobalt 60 the source size are between 1.5 cm diameter to 2 cm diameter

As the SDD increases the geometric penumbra decreases, but as the diaphragm gets near the skin the scattered radiation from the diaphragm results in more skin dose, hence there must be at-least 15 cm gap between skin and diaphragm.

As the SSD increases,due to the effect of inverse square law the geometric penumbra increases.

Source Strength

The Source strength influences the dose rate, the Dose date decreases as the SSD increases due to inverse square law effect

As the PDD determines how much dose can be delivered relative to Dmax, the SSD need to be as large as possible, but as mentioned earlier the dose rate decreases due to distance.

Considering all this factors the SAD is kept at an optimal distance of 80 cm.

In new machine100 cm SSD is available due to more specific activity of source, without changing the source size, it is possible to go for higher SSD not by changing the geometric penumbra.