The Space Launch system is being designed to use initially a modified Delta IV upper stage, but then the Exploration upper stage with 4 RL-10 engines. Why wouldn’t they just use a RL-60 engine which has almost 4x the thrust instead of using 4 RL-10s? And why hasn’t the RL-60 been utilized yet if it has the same isp but 4x the thrust of the RL-10? Stages that use the RL-10 have to pitch up during burns to make up for the low thrust of the RL-10, so why do we continue to use it if a better alternative is available?
The RL60 at a projected thrust of 200-250kN is only twice as powerful as the RL10 (99-110kN in modern versions), so you'd need two of them to replace 4 RL10s; it has a similar thrust-to-weight ratio as the RL10. Thus, for any stage with multiple RL10s, half as many RL60s would make it roughly equivalent, but RL10 has the advantage of being an existing, proven engine.
Both engines use a nozzle structure which is extraordinarily labor-intensive to manufacture. If RL60 had been designed for inexpensive manufacture, it would be in much greater demand, and would be a good candidate for new upper stages. As it is, it simply doesn't offer enough benefit relative to the RL10 to be worth producing.