That is, as it became clear to Cruz, et al. that McConnell was going to be re-elected as GOP senate leader, their choice was to either vote for him and at least remain in consideration for important committee assignments or to stand on conservative prinicples and vote for someone else in which case McConnell who, based on his long history uses his power far more vigorously against conservative members of the GOP than even the most socialistic of the Democrats, would push the conservatives so far to the back of the room as to reduce their influence to near irrelevancy.
I understand the point that under the RINO regime this is likely to happen anyway, but if they did not vote for McConnell it would have been a certainty. Others may argue that the unanimous vote was done as a show of united strength against the senate members of the Democrat Socialist Party, which may be an important reason to do so, but I don't think it's the primary reason.
maybe Cruz didn’t cast a vote at all