score:3
Short answer:
You ask why he didn't pursue. But why would he pursue?
Long answer:
If you read the entire accounting of the battle, you'll find that while defeated and having lost morale, the Gurid army was not destroyed. It has "only" be faced with heavy close-in fighting and its commander was wounded, which means that the Gurid army had no leader to lead it to fight.
However, if Prithviraj had attacked against, he would have faced fierce defense by no doubt. His own army was also probably damaged, given that one of the commander, Govind Rai, was wounded, and considering that Prithviraj was unable to take the fort of Tabarhind for 13 months.
So while I am not aware of primary sources indicating why Prithviraj did not pursue, the Ockham's razor tells us that pursuing the Gurid army was not "a golden opportunity to finish with them, and thus evade the defeat of the second battle of Tarain, and so why did he not pursue?", it was just an opportunity of fighting again while Prithviraj had already gained is current goal, which was repelling the Gurid army to be able to take back the fort.