There is a legend behind this why Lord Sudarshan is still a pole and not shape of deitee.
The story is narrated in the Mahabharata, which states that once in Dwaraka, Rukmini and Satyabhama wished to hear the Gopalila of Sri Krishna from Rohini, the mother of Sri Balarama.
While Rohini described the romantic episode, she asked Subhadra to guard the entrance door.
This time, Sri Krishna and Sri Balarama arrived and along with Subhadra, they listened to the enchanting description given by Mother Rohini.They were all plunged into such a great depth of passion that their limbs started melting.The wheel in the hand of Sri Krishna also melted into a pole shape.
At this moment Narada arrived there. Finding the three Deities in the form of a pole, he prayed to Sri Krishna to retain this shape in Kali Yuga to save the devotees.Therefore, people find Lord Bishnu, His brother Lord Balabhadra, sister Devi Subhadra in pole form along with Lord Sudarshan.
Sudarshan Chakra is also found atop Sri Mandir. Sri Sudarshan, the king of wheels, is 14 ft. in diameter with a thickness of 2 inches, made of an amalgamation of eight metals. The wheel is also called ‘Nilachakra’.There are two circles in the wheel; one is at the centre and the other is the outer.Both the wheel and pole shapes of Sudarshan have different historical backdrops.
How Sudarshan was created is described in Brahma Purana, which states that to get rid of the wicked demons, all the demigods prayed to Lord Vishnu. He asked them to combine all their energies. Lord Shankar shaped this combined energy into a wheel, which he handed over to Lord Vishnu to kill the demons. In another volume, Pasupata Darshan, it is described that the Universe is a wheel and Lord Shankar is moving around, sitting on this wheel. During Rath Jatra, Lord Sudarshan comes first to the Chariot during Pahandi, and placed in Devadalan, the chariot of Devi Subhadra.