More than likely, they're campaigning or fund-raising (or both).
However, note that 96 Senators voted -- Rubio and Cruz plus two other Senators missed the vote, probably Democrats. On votes like this, it's common for Senators from opposing parties to "pair" their votes. Thus, one "Yes" vote and one "No" vote can be away from Senate on other business without impacting the outcome.
I know, but I think it was important for them to be on the record for this since they both want to be Commander in Chief.