probably bc korea's ( and east asia in general iirc ) standard is white skinned and only those who does workouts and heavy jobs outside are tanned
you can try taking a look through this list, but yeah, it's pretty rare
https://www.mangago.me/home/mangalist/290509/
these are decidedly masculine bottoms, but not super muscular, if you're okay with that
https://www.mangago.me/read-manga/love_sick_dog/ (ouroboros from the same author also has a darker bottom, but you should mind the tags. but it's really good!)
https://www.mangago.me/read-manga/capture_me_with_an_expert_kiss/
More like I actually wanna see , is there any bl with the bottom being dark skinned/ tan AND NOT MUSCLY I don’t know why korean authors keep associating dark skin with a buff guy I just wanna see a tanned twink