Why is Connecting Rod Cast Iron?

Cast iron is harder than steel, it won't bend under force.

The shape is also suitable for casting, rather than making a weldment. Sometimes aluminum is used.

all the others plus its cheap

Automotive connecting rods are not made of cast iron. They are either cast steel, aluminum or titanium:

"In modern automotive internal combustion engines, the connecting rods are most usually made of steel for production engines, but can be made of aluminium alloy[citation needed] (for lightness and the ability to absorb high impact at the expense of durability) or titanium (for a combination of lightness with strength, at higher cost) for high performance engines, or of cast iron for applications such as motor scooters." -Wikipedia