It's probably the same reasons a lot of long-time green card holders in the US don't apply for US citizenship. Applying for another country's citizenship can be daunting (even if it is "simple" it still involves many steps), expensive, time-consuming, and possibly have an uncertain outcome (what if it doesn't work)? For a lot of people, citizenship is either something they don't worry about at all--because what they have works for them--or something they worry about very much because what they have doesn't work and they can't get the one that does.

This gentleman likely fell into the former category and had more important, to him, things on his mind than "what does this piece of paper in my file cabinet say about where I'm allowed to live." Besides, he may have firmly believed all of the things said by the British government about how "Brexit won't change the status of settled EU citizens" and thought that applied to him and so on.

It's easy to dismiss his concerns without having lived his life and I don't think we should do that.