They don't hand out Nobel prizes for glossed over topics.

Leaving aside the fact that the Economics prize isn't actually a Nobel Prize, topics which historically haven't been given enough attention are exactly where the highest impact research takes place.

If externalities had always received the attention they deserved, Coase would have never received his prize, because his work would not have been so important.

Coase did his most relevant work in the 1950s, and it wasn't as if he invented the idea of externalities. It was first given serious academic weight in the 1890s, and Pigou created the concept of externality correcting taxes in the 1920s. His work was important because it proposed a more market based solution than Pigouvian taxes.

I think its safe to say that externalities are not, and were not, an ignored sector given more than a century of serious work, and the fact that it is covered in any intro level Econ course.