Welcome, prounix!  I am not entirely certain about the answer but it may have been due to resentment against their father and his unfaithful ways.  But having read briefly about the unmarried daughters of Louis XV, it seems that marriage options would have been limited for them anyway.  If the marriage wasn't politically beneficial, or acceptable by royal standards, it might simply be an impossibility (as was the case for Princess Henriette-Anne).  This may go against the conventional idea that the rich and powerful had limitless power.  The royalty were limited by what they could do with their love lives, with royal women being much more constrained than royal men.