I'm not sure, if that is the reason.
Optics is one of those areas that you can get lots of kids to 'gel' with, since the practicals are fairly simple, and systematic. It is a good one for the kinesthetic learners in the group, good to bump up the D / C grades.
Because they can literally 'see' what is happeneing, they 'get' it. And the maths just follows on, and is, as you say, quite basic.
The tendency seems to be that the exam boards are keen for kids to look at the 'big picture' sciece topics. Now, I can understand this because the big picture stuff is fun and also often relevant to evry day life (gene testing, or CO2 neutral fuels for example). What they fail to understandis that if the kids are really going to understand this stuff in a meaningful way, they have to understand the basic science first.
Biggest problem that I have, at all stages, is that the curriculum time is so short, and we have so much 'stuff' to teach them, that we don't have the time to teach them scientific method and process.