The liquid-fueled rocket would have the same lift capacity as the solid, so the solid has no advantage. The survey is operating under the premise that they have comparable capabilities. Then the distinguishing features between the two choices is what is included in the survey.
Both designs are to meet the same requirements for the customers. And it is design to requirements. The rocket is not made better or worse than specified. Should a design prove to be overperforming in the simulations, its performance will be reduced in the next iteration while increasing quality or reducing costs. And that until a launcher design is there, that does exactly what it is meant to and do that in the most effective way, with efficiency meaning in first place : costs, reliability, costs and agility, in the sense of: How long it takes from signing a launch contract to launching the satellite.
The Ariane 5 is pretty poor there, since it usually has to wait for two fitting satellites to be dual-launched.
But the problem was the manned spaceflight capability for the liquid-fueled version was not even mentioned as part of its advantages in the ESA discussion between the two.
Yes, because the Ariane 6 is, as I have said above, especially and explicitly designed to be a cheaper launcher as the Ariane 5 for small and medium sized geostationary satellites with higher flexibility than the Ariane 5.
Why should they investigate which is the better manned launcher? Maybe they should also investigate which of the two looks better as Revell modell? Or which has the smaller CO2 footprint?
If it were mentioned then a full discussion could be made about the additional costs for a manned system for this liquid-fueled Ariane 6. Given its smaller size, and fully liquid-fueled design it would likely be much less than the estimates put forward for the Ariane 5. Also, as the example of the low cost of man-rating the Atlas V shows, the man-rating of the launcher aspect can be relatively small.
If it were mentioned, the project would take 30 years before being cancelled without any bit of hardware being produced. You would have too many requirements to investigate in the first phase, too many engineer hours wasted in meetings, because you try to do it all. And even in the best case, you would only get a compromise, an expensive satellite launcher and a poor manned rocket.