A first term Senator does not have the seniority to be Majority Leader. Very few Senators would vote for him and it would be a humiliating defeat. Depending on how vindictive McConnell is, challenging him could have resulted in Senator Cruz being relegated to committee assignments that gave him little or no real impact.
I don’t get this whole “junior, senior” nonsense. Seems to me all Senators and Congressman are equally elected by the electorate and there shouldn’t be a situation where a “junior” member has to kiss ass of a “senior” member. All of that stuff seems to have nothing in common with the founding documents and has just become accepted practice.