If simulations show that it's not currently worth the effort, but they want to keep the option open, that implies that they believe some technological change in the future may make the relaxed memory model bear fruit.

Actually, now that I think about it, another option is that the weaker memory model is more advantageous in lower power implementations, like iWatch, and preventing developers from relying on the stronger memory model available in M1 reduces the number of bugs in iWatch applications that share code with iPhone apps or OSX apps.