Rajarata was abundant due to regular wars for the Rajarata land Sinhalese kings tried to re established Rajarata but they werent allowed by Pandyan empire They had full control of the Sri Lanka in  They got real opportunity after  years from the Tamil invasion That time people adjusted to wet zone so it is possible that descendants of the engineering families didnt had the practical knowledge to rebuild the dry zone hydraulic system It wasnt that much simple Even British couldnt understand how the system works  So clear major reason is they were adjusted to a another system Also non existence of expert knowledge to rebuild the system could help to not to go to dry zone again  But clearly it is not enviornmental issues It was run more than  centuries I dont see Magha is irrelavant here Magha doesnt mean only Magha He couldnt do all the things alone He was supported by his army Even king didnt had child army had children Those are the people who came to Jaffna      pre History of Jaffna kingdom   Jaffna was part of Rajarata before the collapse of Rajarata Some people trying to say there was independent kingdom convergence Jaffna But how there can be a kingdom without people  People came with Maghas army So due to what it could survive from the power of Rajarata       Starting point of Rajarata  Magha is not the one who established Jaffna kingdom Magha is the last king of Rajarata People accept Magha as king as they couldnt find any royal blood at that time and Magha was Nissanka Malla relative Also sinhalese power was very weak at that time due to regular providing of strong armies to assist Pandyan to servile from Cholas Due to this Veera Pandyan a Pandyan prince who get help from Sinhalese and couldnt servive chola powers invaded weak unstable due to internal conflicts for kingship Pollonnaruwa Then Kalinga Magha Sinha invaded Rajarata with army of Tamils There was a battle Magha defeated Veera Pandyan Magha was accepted as the king But it wasnt sure whether Magha was controlled by his army chiefs Armies destroyed Buddhist stupas may be to find treasures Also there wasnt way to feed large number of army Therefore they expelled Sinhalese from north People go against the king as they did to Mahasen and fight against king Magha had to abandon the Rajarata This army wasnt big as previous chola armies Previous occasions Rajarata rulers completely expelled previous chola invaders and those who wished to join  king was also allowed  vellakkara group But Maghas army had good support from Pandyan It is not clear why Pandyan betrayed Sinhalese as Sinhalese regularly provided armies to survive from Cholas Pandyan was just near Jaffna and able to respond very quickly After repulse of Sinhalese Maghas descenders had to accept they obey Pandyan King Pandyan ruledassist jaffna with military family called as Aryachakravarthi But in  powerful Muslim empire destroyed Pandyan capital and captured the king After Pandyan was ruled by Muslims They didnt care to assist Jaffna At this point Aryachkravarthi him self made as the king of Jaffna  It is not right to say Magha as the establisher of Jaffna kingdom He was the king of Pollonnaruwa but after his death and tamil forces was expelled from Rajarata core areas to Jaffna they were under protection of the Pandyan king So Arachakravarthi established the Jaffna Kingdom