Basically when Ariane 5 was designed, the Shuttle was all the hotness and the European were mostly copying the design.
And for the Ariane 6 they didn't have time to come up with anything new. Most of the Ariane 6 are just things that were already in development for the Ariane 5 ME.
It takes Europe 10+ years at least for a new engine. So Ariane 6 couldn't have used anything new.
As for why not using it on the second stage, its simple. Its to big.
Ariane 5/6 are late staging system. Meaning the boosters provide a lot of the load to get of the ground. The first stage flies was longer then in systems like the Falcon 9. So the hydrogen is bad for takeoff but if you stage late, hydrogen advantage matters more.
And for the Ariane 6 they didn't have time to come up with anything new. Most of the Ariane 6 are just things that were already in development for the Ariane 5 ME.
It takes Europe 10+ years at least for a new engine. So Ariane 6 couldn't have used anything new.
As for why not using it on the second stage, its simple. Its to big.
Ariane 5/6 are late staging system. Meaning the boosters provide a lot of the load to get of the ground. The first stage flies was longer then in systems like the Falcon 9. So the hydrogen is bad for takeoff but if you stage late, hydrogen advantage matters more.