We have 2 top quality CBs. Martinez is brilliant at LCB, Varane is brilliant at RCB but will miss some games with injuries. So you need a capable squad player (Lindelof) to cover games where he's injured and you need a young CB with potential to be marked as the long term replacement (lower the age of the group and replace Maguire). Kim is far to good/experienced for either of those 2 roles, so it never made sense.
In what way is CB a priority? ST, CM, GK are the 3 priorities, and that's been repeated by most sources too.
Of course his release clause was a good price but roughly 50m is still big money and it's roughly what we can afford to spend on each of GK, CM and ST this summer. We don't have the budget for a 50m CB, along with the wages he'd then command. But mainly it was just the squad roles aspect.
Having 2 world class CB compete for the same position given weaknesses at GK, CM and CF wouldn't make sense, CBs don't rotate much in general, and we have a very good deputy for Varane anyway, and can cover Martinez really well with Shaw and have Dalot or Malacia cover LB (or the young kid who came back from loan from Preston). We can sell Maguire and not touch our defence and be pretty much fine in defence unless someone picks up a serious injury. We can cover normal absences as it is.