What exactly is the appeal of the KS60? I see it's 5.3kg (11.7lb) lighter, which would to me be the biggest reason if you move saws around a lot. But to be fair the KS120 is ~50lbs which isn't too bad.
If you aren't moving it around all the time or lugging into houses, I'm struggling to see the use for it. Maybe this is the basic answer, maybe most folks in N.A. are like me and look at the situation as "why would you get anything but a 12" sliding compound" basically why wouldn't you get the biggest, most capable one you can. I'd think the only real constraint would be moving it around. I can see if you say are a flooring installer and just need something to do small cross cuts, but then why not just by a small cheap saw (granted dust collection may be a valid reason why).
I'd overall guess that Festool has looked at the N.A. market and the answer is like many products where the N.A. market isn't exactly space constrained. Big houses, big vehicles, etc. So making a product a bit smaller/lighter may be appreciated by some, it's going to get shrugged off by most folks.