The EU is mostly stringent on the environment, and single-use rockets are pretty bad for the environment: all the metal and sophisticated parts must be produced, only to be thrown into the ocean.
I would expect reusability to be a requirement based on that factor alone. You ca produce carbon-neutral fuel, but there is a lot of environmental load involved in production of the very rockets.
reusability is mainly done for cost instead of environment, but you need to launch a lot to re-cup the costs, and esa is not a commercial org like spacex.
I do think the ESA rockets use a more eco friendly fuel then others, that is probably more important then some metal on the seabed.
I would expect reusability to be a requirement based on that factor alone. You ca produce carbon-neutral fuel, but there is a lot of environmental load involved in production of the very rockets.