USA and UK and France dont do SSKs because the fuckers dont have the manpower to crew the bloody things. SSNs are absolutely disgusting waste of money for standard duty operations.
France and RN are INCAPABLE of doing your normal submarine mission profile of say, attacking the enemy's merchant shipping, They're dedicated SSBN escorts.
For everything else, be it interdiction of enemy merchantmen, for sulking around and gathering SIGINT and laying mines, and inserting SF - SSKs are the best thing.
SSNs are noisier, more expensive to build and operate and the crew qualification requirements are off the charts. Its a stupid bet for non escort SSBN missions.