Many of the reasons for Why do some professional cameras have dual memory card slots?Why do some professional cameras have dual memory card slots? seem to apply to consumer interchangeable lens cameras (whether mirrorless or SLRs) as well:
If one SD card becomes corrupt, I want to continue shooting to the other one.
I may want to shoot a burst faster than one card can handle.
I may have two small SD cards. Rather than buying a bigger one, or carrying both and swapping them, which is inconvenient, it would be good if the camera let me combine the capacity and speed of both cards.
An SD slot takes up a tiny fraction of the room in an interchangeable lens camera. Maybe the manufacturers want to artificially differentiate consumer cameras from pro models, but nobody looking for a $600 camera will buy a $3000 camera because the latter supports two SD cards.