Man rating seems like an obvious thing. Safe enough for manned flight.
But in reality, there really was/is no standard for it. Some things are generic. Sufficient (Usually triple) redundancy in flight computers/controls.
SpaceX has cited building to 1.4X structural margins expected, instead of 1.25. (This is a hard one to retrofit in after the fact).
NASA actually came up with an attempt at a standard. Thing is, Soyuz does not meet it, yet has flown how many hundreds of manned flights (And 1700 total flights for the booster).
Per the Wikipedia entry on Human Rating Certification NASA in 2008 came out with a standard for the new entrants.
(NASA) has published NASA Procedural Requirement NPR 8705.2B - Human Rating Requirements for Space Systems, defining the certification process and a set of technical requirements to be applied to its crewed space systems in addition to the standards and requirements that are mandatory for all of NASA's space flight programs
The Shuttle would not meet the standards either. Not sure about Apollo/Saturn V (and NASA would probably very much like people NOT to ask that question, I suspect.)
So asking why were Atlas V/Ariane V are not human rated is hard to answer. Mostly because until recently, human rated did not have a very hard target to hit.
Obviously no one plans to build an unreliable rocket. But obviously as well, this is still rocket science and it is hard. Additionally, if you believe in a man rated standard, a large number of successful flights alone does not prove it is man rated. But if you are NASA you get to ignore the rules (Shuttle/Soyuz) at will, if it is expedient. Which leads to the conclusion that man-rating is sort of tenuous at best.