It's certainly possible that every J seat is sold, and many passengers are awaiting a seat assignment. There are absolutely plausible scenarios that could result in that being the case. But it's unlikely.

It's also possible that Y and/or W are oversold. But that's also unlikely.

The most likely explanation here is that revenue management has a much more intricate reason for restricting J inventory at this time, and you'll probably never find out what that reason is. I'd bet big bucks that you'll see additional J open up at some point prior to the day of departure.