Karna says that people will think that he switched sides because he is afraid of Arjuna
Who does not suffer when Dhananjaya is aided by Krsna? If I now go over to the Parthas, who will not think that I am frightened?
- 144, Karna Upanivada parva, Mahabharata.
Karna says that he can't abandon the Kauravas suddenly after they have given everything to him and after he had given them hopes of victory
The sons of Dhritarashtra have given me a share in every object of desire. They have honoured me and given me every kind of happiness. How can I act counter to that now?They have always served me and have always bowed down before me, like the Vasus before Vasava. They now confront a feud with an enemy. They think that with my life, they can withstand the enemies. How can I act against the desire that is there in their minds?
- 144, Karna Upanivada parva, Mahabharata.
So his two reasons were -
A. Fear of being called a coward.
B. Wanting to repay the Kauravas for their helps.