There are few, if any, fixed costs associated with a basic Steam release; achievements might cost a little more but wouldn't take that long in dev time.
I imagine it's more 5pb. just not understand Steam and frankly being afraid of exposure; a lot of these VN companies are just completely, blindly terrified of the overseas market in any form, remember. After some of the bad press VNs have gotten over the years, it's a struggle just to get them to even try, because they'd rather just stay home and make money domestically and that's it. Trying to get them to commit to a business model that is almost completely, legitimately alien to them - Steam mass digi-distro - is an even greater hurdle. It's one I've had to climb before in a somewhat different capacity.
Wonder if I should point a certain Ms. Love JAST's way, see if she can help convince 5pb. on the viability front at all.