It is likely this was intentional. If you don't want to be present for a vote for some reason, you can find someone else who would vote the other way (usually reaching across the aisle) and both agree not to participate. Happens all the time.

Rubio is too busy campaigning to actually do his job.

> Nay - Sen. Bernard “Bernie” Sanders [I]

If he had something to gain from voting he would have, but he can use the excuse that he is campaigning (even though that's not much of an excuse).

Not voting for the bill is effectively a "no" vote.

Senator Boxer is retiring after her current term, so I doubt that cowardice explains her non-vote.