Only 46 candidates nationally did FS1 and FS2?

1. The 2 modules are always harder than the 1 modules (hence why so few people take the 2 modules in general)
2. A lot of people who do further maths take physics, so further mechanics is an obvious choice because they already understand the concepts involved
3. Similarly, people have been doing pure maths for longer, so further pure is again a good choice in a lot of people's eyes because you already have a good foundation

Perhaps more concerningly, why do you want to take two 2 modules together -> this would mean you have to take at least four optional modules, when you only need two (you would have to take FM1, FM2, FS1 and FS2 if you did what you are suggesting)

I'm taking FD1, FM1, FM2, FS1 only.

Is this international A levels? That would be very unusual for UK A levels