The main reason behind KaalSarp Poojas being held only in Ujjain is based on the facts written in Sankadh Puran- One of the Holiest books of Hindu Mythology. When the Devtas and Asuras were fighting to seize the things which were coming out of the ocean, those things were being distributed in Ujjain only. From all those things, the most important of all was, Amrit. For Amrit all the Devtas and Asuras were Arguing the most and at the same time, there was an Asura by the name Rahu, by deceit he drank some of the Amrit because of that Lord Vishnu became furious and used his Sudarshan Chakra to split Rahu into pieces, but because of Amrit the bodies didn't die and stayed alive and then later the first part of the body was called as Rahu and the second second part was called as Ketu and then happened their rebirth but on different Nakshatras. For Rahu, it was Bharni and for Ketu, it was Ashlesha. The Lord of Bharni Nakshatra is Kaal and the lord of Ashlesha Nakshatra is Sirp(Snake).

That's why from, Kaal + Sirp- It became KaalSarp.

This is the main reason, why all the Rahu Ketu related issues are concerned with KaalSarp Dosh.

The name of Vishesh Rahu ketu's mother was Sihinka. She was the younger sister of Hiranyaksha and Hiranyakashyap and she was the aunt of Prahlad. Rahu was born from Sihinka and that's why he is also called as Sihikey.

Abhimanyu the son of Arjuna and the family memeber of the truthfull Pandavas. Abhimanyu's grandchild Janmejay also did a Pooja in Ujjain. It was held to take revenge for his father's death, 55 kilometres away from Ujjain but after a certain time that Pooja was stopped by the Name of Lord Mahakaleshwara because if it would not have been stopped, all the snakes from earth would had been vanished that is also one of the reason KaalSarp Pooja is held in Ujjain only.

How does KaalSarp Dosh arises in someone's life?
Whenever all the planets gather in between Rahu and Ketu. So, on that circumstances Kaal Sarp Dosh arises in ones Kundli or else if there is any week planet like Moon or Saturn, which is outside of Rahu and ketu then this also increases the chances of having Anshik KaalSarp Dosh.

There are mainly 12 types of KaalSarp dosh.

Anantha KaalSarp Dosh
Kulika KaalSarp Dosh
Vasuki KaalSarp Dosh
Shankuphala KaalSarp Dosh
Padma KaalSarp Dosh
Mahapadma KaalSarp Dosh
Takshaka KaalSarp Dosh
Karkotaka KaalSarp Dosh
Shanchachud KaalSarp Dosh
Ghataka KaalSarp Dosh
Vishdhana KaalSarp Dosh
Sesh-naga KaalSarp Dosh

Those people who has KaalSarp Dosh in their Kundlis, most of the time they feel down and low on their field of work and because of these Doshas , they never get success easily. whatsoever kind of work they do their luck never seems to favor them. Be it in the field of Job, Marraige, love, Money and Health.