Mostly because the target market usually doesn’t value dual slots enough to justify the added cost, space, and complexity.

For consumer cameras, card redundancy is less critical: if a card fails, most users can stop and swap cards without major consequences. Dual slots are much more important for paid or “mission critical” work where losing images could mean losing a client.

They also usually don’t improve burst shooting on lower-end cameras. In that class, the bottleneck is often the processor, internal buffer, or SD interface speed, not the presence of only one card slot.

Using two small cards as one larger/faster pool also adds implementation complexity, while simply swapping cards is good enough for most buyers.

Physical size matters too: a second slot, door, controller logic, and internal layout are not free, even if an SD card itself is small.

So while product differentiation plays some role, the main reason is market demand: buyers of consumer cameras generally prefer lower price or other features over dual card slots.