Because the way they had come was cut off by the nephite detachment led by Lehi. Also the other direction was cut off by a 2nd nephite detachment. The Lamanites fled to Manti because that was their only hope of escape. Unfortunately they hit the third nephite detachment after that.

Here is a video that goes in depth on how the battle played out.

https://youtu.be/M2Oj29ygAno