Does not seem that hard to understand to me. They led the round, that means other investors are in on the deal as a result of Sequoia. If they had taken the money back, they would have lost the trust of other investors in future fundraising rounds that they lead and left the other investors in Finix in a bad spot with a lot of capital in a company that is now, presumably, $21m short of their needs.
$21m is nothing compared to the loss of trust in Sequoia that pulling the funding would have caused.