It mostly comes down to a few factors. In the past I've asked Arrow, Plain Archive, Kimchi and Nova about various titles and almost every time the answer is '[insert studio name] owns the license for it and it's not something they want to do'
Even if the retailer wants to do something people have asked for sometimes studios can't be bothered or just make it difficult (agreeing artwork for example). I'm sure you're also aware that companies deliberately 'vault' some titles so that probably works into what comes out. As much as Steels are close to our hearts they're still pretty niche to a Studio. Also, retailers will only bother with premium releases of things they know will sell. Why waste their effort on 'classics' when they can do something on films that are coming out now. It takes a lot of time and effort behind the scenes to get premium release sorted end to end. Plain Archive are the exception in this case as they do a lot of niche titles (just not in steels) but they have to jump through hoops sometimes.
As for this film; Res 6 was made for $40m. It's made just over $300 in total. The US gross was only $26m whilst this made almost $160m in China alone. As this movie was being released anyway, Kimchi were always onto a winner with this. As far as I know this is the only one of the Res films to get a Premium edition. Res 5 didn't anywhere. Personally I love the film. I love the whole series (it's made well over $1bn) but that's just me and I get why LOADS of people hate it. I personally don't know anyone else that likes this film aside from me and the guys in this thread. Kimchi as an Asian retailer are always going to look to take advantage of the demand in their own region.
Sorry for the long post.