Although I’m not sure you actually want reasons, I’m interested in the topic so here are the reasons I have opted to use plastics (of various kinds) instead of stainless steel in the past.
The biggest one by far is making them disposable this was HUGE when I was working in sterile manufacturing. The evaluation was done between the cost of maintaining, cleaning, and sanitizing a 316L SS bioreactor unit (very similar to a hash washer) or going instead with disposable components that came “pre-sanitized” and could be recycled instead of maintained/cleaned/sanitized, etc. The price difference was HUGE - it was way better to do that than to go forth with the stainless option.
The corrosion/rouging, pitting, scraping, leeching from 316L SS was a big deal, just normal wear and tear. Moving away from that and into insulated bags or buckets that were single use, BIG BIG economic savings. Add onto that the cost of cleaning and sanitizing (which our industry is just barely starting to care about) and going with a “bucket” or a “bag” that you can take out and recycle is huge.
I don’t know which system you are looking at - as you didn’t mention on and the price you mentioned is more than most of the plastic units I have seen and way less than all the other systems I have seen that are not just a metal bucket with a person stirring… -shrug-
I’m interested to know why your specific interest is “micro-plastics” from possible abrasion. Are you thinking these are micro plastics more than is normally found in water and ice in the US? Are you thinking that this would be such a high level of impurity that it would cause harm?
Is tank/bag in question easily removable and replaceable and is the replace seriously cheap? Like less than $100? If so - I would learn towards the easy to clean/sanitize direction. That’s how I have designed most of the filters/hoses/bag in tank systems I use. However - I’ve also been at places where the local AHJ wanted to see stainless, so that was my only option.
Another big reason for me to use plastic has always been WEIGHT. Meaning the size of the lift or the person moving the thing around doesn’t have to be as large. And that has also really mattered for mobile units that I was preparing, where I had to stay under a specific weight limit, and other things like chillers / heaters / raw materials were more important to have.
Occasionally, I’ve seen systems made from plastics that are coated with lots of different things specifically because they came from other systems where material compatibility was important and using glass or metal would have been incompatible. I’m thinking mostly of chromatography stuff - for when you are working with things that corrode/etch SS and glass.
Without knowing their full design - I could also consider that its possible someone in their engineering department thought, “Does plastic sweat less than SS? I think so…” and then decided that because of that they would need less insulation. Because plastic doesn’t conduct temperature as well… would that be a good thing? Perhaps not if you are trying to cool it with a recirculating pump or something, but maybe it would be okay if you were just dumping ice into it. Maybe. -shrug-
Anyway - sometimes there are good reasons for using plastic beyond potentially being cheaper. Its also worth noting that at this point food grade plastics are almost the same price as their SS counterparts. I can get a 500g SS tank for around $2k - I can get a 500g heavy wall plastic tank for around $2k… you know what I mean?