I’d guess to differentiate the civil / uncivil dichotomy from the civil(ian) / military dichotomy.

That said, I feel that civil engineering is fairly widely understood to be a subdicipline of engineering.

In the US/UK civil engineering is often taken to mean constructing buildings (very roughly, in a nutshell), whereas other countries (in particular continental Europe) use the meaning that you quote. So you can have a Belgian civil engineer who specializes in theoretical physics, and knows more about the Higgs boson than about "traditional" engineering topics.

In Belgium, there are 'civil engineers' that know nothing about the construction of roads buildings or bridges, but are specialised in electronics, computer science, thermodynamics, ... .

In the case of Lemaitre, he was studying this: https://en.wikipedia.org/wiki/Mining_engineering