Why has the UK and France persisted with this approach when there are many conventional options like Super 40, for example?
Quite simply, it is one of space, although additional armour piercing performance is always a good thing.
Modern vehicles need modern electronics, and contrary to popular belief, modern electronics, at least in combat vehicles, are not getting any smaller. Modern vehicles also need modern people, and modern people are larger and wear combat body armour, we also can no longer insist armoured vehicle crew are small in stature. All this places a premium on internal turret volume, so anything that reduces the volume of one of the main turret components simply means more room for ammunition, electronics and ergonomics i.e., a good thing.
This comes at a cost however, the CTAS is expensive (regardless of arguments about cost per stored kill) and unless others purchase it, only in service in relatively small numbers. The burden of ongoing qualification and development will fall disproportionately on France and the UK. There are wider arguments about the concept of operation for armoured infantry and reconnaissance forces and their need for such a high performance, and expensive, weapon, but they are outside the scope of this document.
Regardless of whether anyone thinks the CTAS is a good idea or not is irrelevant, to coin a phrase, we are where we are.
The question the UK faces is whether to double down, or carry on with the CTAS in service on just two vehicle types. In order to maximise commonality, realise economies of scale and provide confidence and impetus to an export campaign.
Export customers mean shared development costs and lower ammunition costs.