why is potassium the most free element in most of plants
There are many functions related to potassium some of which are shown below

Potassium regulates the opening and closing of the stomata by a potassium ion pump. Since stomata are important in water regulation, potassium reduces water loss from the leaves and increases drought tolerance. Potassium deficiency may cause necrosis or interveinal chlorosis. K+ is highly mobile and can aid in balancing the anion charges within the plant. Potassium helps in fruit colouration, shape and also increases its brix. Hence, quality fruits are produced in Potassium rich soils. It also has high solubility in water and leaches out of rocky or sandy soils. This water solubility can result in potassium deficiency. Potassium serves as an activator of enzymes used in photosynthesis and respiration[4] Potassium is used to build cellulose and aids in photosynthesis by the formation of a chlorophyll precursor. Potassium deficiency may result in higher risk of pathogens, wilting, chlorosis, brown spotting, and higher chances of damage from frost and heat.

and the answer to your question is the small size of potassium when compared to all the other cations in plants 

another reason for this is its association with the chloride ion .
chloride and potassium ions will be playing follow the master game in which if any cation takes chloride ion near it then potassium will also follow chlorine and the same is the case with potassium whenever any other anion attracts it this makes potassium highly mobile.  source wikipedia

hope you got your answer. if any doubts left feel free to comment