Practical, since they could be adapted for all kinds of uses, from hard outdoor labors, to indoor pursuits, like house keeping.
Inexpensive, since yarn was at that time cheap, and easy to get, especially if you had sheep of your own, so you could spin your own yarns, and save a lot of money.
And of course their warmth, since the unique qualities of wool fibers, make them ideal for outdoors, or cold, drafty, cottages. Wool, because of its fiber structure, creates a nearly impenetrable surface when knitted, woven or felted. Add to that the presence of natural lanolin on the fiber surface, and its water repellent as well.
Slowly but surely, other knitted things crept into the households of the well to do. First in the form of knitted mitts to keep the hands warm in those aforementioned drafty homes. Next, interestingly, night caps, often heavily decorated with embroidery and beads.
But it wasn't really till the 20th century that the sweater as we now know it came to real prominence. Along with the easing of all kinds of social standards, and the increase of people of all walks of life engaging in sports, sweaters, and sweater dressing, began to rise in the public mind to a level where it crossed all class borders.
It was really two factors in the early 20th century that pushed this forward successfully. One was the rise of skiing as a luxury winter sport. It was expensive to go skiing, since it required specialized equipment that wasn't easy to come by, travel to remote places, and the freedom to do so. So inevitably the attire connected with it, gained cachet, especially the boldly patterned sweaters.