since a vote for KXL would have presumably, as you say, "saved her"... along with 'letting the vote happen', why didn't Reid work to ensure/secure her that final vote?
Because he has competing priorities. He's trying to walk a tightrope between the Dumbocrats who have to vote against KXL or be abandoned by their supporters, and Sen. Landrieu, whose constitutents mostly favour KXL. He hoped that by bringing the vote, she could vote in favour of KXL in an attempt to gain support in Louisiana, whilst the other Dumbocrats could vote against it in an attempt to gain or retain support in their own states.
That and, as petros said, he can't. The Senate is a notoriously unruly body. Not having the Brit/Can parliamentary system, our politicians are much more independent.