Perhaps they ran into some issues that they are having difficulty fixing. Possibly they are still working on the low memory issues. I do find the opt-in release quite usable though. I'd rather have software released based on progress than a date.

"The system's handling of out of memory (OOM) conditions still needs some work, meaning

The device can behave sluggishly when it is low in memory. This can cause annoyances during heavy daily use of your Jolla. A reboot makes things better, but we understand that this is not something our users want to do frequently.
Common tasks like answering calls, opening notifications, multitasking, switching between apps etc can take tens or hundreds of milliseconds to operate in low memory conditions. For a smooth operation, the OS needs to makes sure that the memory is never critically low.
Communication between Sailfish apps and background processes (daemons) can take longer when the system is swapping in low memory conditions. This is experienced as stutter in the UI and in extreme cases, it can freeze the UI completely."