During the last ten years all 100 or so launches by Atlas V and Ariane V together have been successful. (One Atlas V payload entered too low orbit, but that would hardly have risked the life of a crew). Why is that not enough for them to be man-rated and carry crewed capsules?
Are there requirements on technical details which overrule actually demonstrated performance? Or is there something else than reliability which plays a role, like higher G-forces than a human can take?
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.
Space.com answered this very question:
The emergency detection system needs to be finished, for example. And a human-rated Atlas 5 will use a two-engine Centaur upper stage rather than the single-engine version currently in use, Patton said, so some more tweaks will be needed to accommodate the change.
There's a good list of other things, but the bottom line is, it's reliable enough, it just needs a few additional tweaks that a man rated spacecraft requires.
For reference, the reason why these types of things are required is a flight profile has to be used that is fairly flat, so that if there is an emergency there will be enough time to slow down before impact. Any reentry vehicle if launched straight up will burn up on its way down, they aren't designed to do that. So the upper stage has to have enough thrust to allow for a relatively flat trajectory, which takes Delta and Arianne out.
For Ariane 5, the reason is simple that ESA don't have a sovereign manned program.
The rocket was initially planned to be human rated.
It has been be quoted as one of the reason why the rocket is so expensive, despite the requirement being dropped.
A bit of an update with this answer: The Atlas V is near the end of the process of being human certified by NASA. The final flight test of Starliner (the crewed flight test, or CFT, currently delayed to late in 2023 at soonest) is required to be completed before that certification can be issued. After a review of the data from that flight, Atlas V will be human-certified.
