[spapin]

There are few reasons:

1 - Engineers and leadership didn't think landing would work that well

2 - The culture at ESA and Arianespace is to NOT have any failures. So they don't have the liberty to launch rockets that spectacularly explode. Even when experimenting, they are extremely risk averse.

3 - The same company builds ICBMs, so demonstrating reliability in civil launchers also asserts reliability for military equipment.

4 -The focus was reducing costs of Ariane5 and make a launcher with similar capability and cost to Russian Soyuz. I don't think SpaceX was the focus for the competition. They also wanted to get rid of the dual launches of Ariane5 and provide a launcher that could just do the large payload of those dual launches independently