Synthetic yarn (NB: NOT ‘wool’) is sometimes cheaper, and some people with allergies to wool or lanolin find it safer to wear next to the skin. Vegans also tend to prefer it.
There are two basic types of synthetic yarn: petroleum-based, such as acrylic or nylon, and plant-based, such as rayon or cellulose.
Petroleum-based yarns, as you might imagine, are not eco-friendly or renewable. They are often flammable if not treated with kemikalzzzz, melting onto your skin. Every time they are washed, they lose tiny microfibres which make their way into the oceans, just like those banned micro-beads in cleansers. Plus, being basically plastic, they degrade like plastic bags: good news for archaeologists, not so good for the environment.
Plant-based fibres will bio-degrade, but the process of creating them can be polluting. For most, the process involves blending the plant material into slush and extruding a thin fibre from that slush, which uses electricity. The process also involves the dreaded kemikalzzz, some of which are genuinely nasty. There’s also significant waste which needs to be dealt with, preferably not by dumping it in a nearby river, which does happen too.
Other, more ‘natural’ plant fibres are not much better. Cotton is famously bad for the environment, both in the growing and the processing. Linen requires the flax plant to be rotted under water for a period of time, then washed repeatedly to extract the phloem (IIRC: it might be xylem, or both). On my family farm, there is a stream-fed pond we call the linsteep, where my ancestors performed this task. It’s now an amusement park for our ducks, and a handy watering-hole for our cattle, but when it was in use, we – and our down-stream neighbours – had to drive our animals to other water sources. The process for making hemp yarn was similar.
Wool is renewable, bio-degradable, non-flammable, need not use the genuinely nastier chemicals in processing (check out the suint method – the residue can be used as a natural fertiliser, which apparently works wonders on tomatoes), all while the sheep mows and fertilises your lawn, provides milk which can be made into ice cream, lanolin for your beauty products, and in the end cuddly sheepskins and yummy meat. Plus, wool is warmer, waterproof, dyes easily with no more than food colouring and vinegar, and when you’re bored with your sweater, you can felt it into pot-holders or cushion covers, or cut it up without unravelling to patch other clothes or make new ones: