You can save space. In such small cases like smartphones, such slots make up a lot.
By the way, I’d like to add another requirement: very robust (IOW: industrial grade) connectors. I just have problems w/ my toshiba notebook where all USB connectors now have loose contact. Smartphones usually have only one - and if the usb slot its broken, you can the throw it away (I doubt anybody seriously replaces usb connectors in smartphones)
The logic is already in the SOC, so it doesn’t matter anyways. The only external parts are PHYs and slots. I doubt (haven’t checked) that combined slots are more expensive than two separate ones (actually, should be way cheaper). And I’d guess there’re even combined PHYs, that even can detect the card type - maybe even 2 in one chip, or integrated in the socket.