You are correct that the market is mostly 6" devices, that being said we also see multiple manufacturers releasing “compact”/mini devices now (according to the analysts apple started it and Sony is rumored to be releasing a true small device soon too).
For me it is the lack of choice and the lackluster specs of the newer devices Jolla chose to support, I got an Xperia X not happy about the size “upgrade” but the X at least was still a manageable size then we got the XA2 and the 10 both larger then the X while not offering anything worthwhile as far as specs went over the X (the X was a top of the line device when it was released - just shy of flagship - while the XA2 and the 10 are midrange devices that never truly offered anything beyond what the X offered).
Personally I hope we either get Xperia 5ii support or the next compact device (and TBH I hope the second more because the 5ii isn’t all that compact), SFOS is all about gestures and single handed usage these things are kind of impossible when the phone gets too large and I also don’t walk around with a purse so I want the device to fit in my pocket without becoming a convenient target for pick pockets or a fall risk.
(As an alternative I also would not mind a Nokia device again having SFOS )
/Edit - should I need to buy a new device today I would buy a new Xperia X even though Android support under SFOS is crap for bad reasons because it is superior to the XA2 and the 10 and also cheaper, if there was a model available that was actually superior or offered a new valuable feature (eg. 5G once it is worth it as far as network coverage goes) I might consider a newer model.