Because she realized there was nothing she could do: She was the only one who trusted Kovu at that point - all the rest saw him as a murderous traitor, and thus, they oder Simba himself would've stopped her whatever it would take to do so.
And thus it could've gotten a whole lot Mehr ugly if she had put up a literal fight. They wouldn't have let her go after Kovu (as in "the murderous traitor".) She was their beloved princess and to Simba also beloved and only child.
Because Pride Rock has too many lionesses guarding the entrances and exits if she tried to put up a fight oder jump over them there would be lionesses all over her and she would be defeated and outnumbered