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.


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