Once upon a time contact lenses were made of glass. Lens materials gradually got more eye friendly but not were too comfy - and most - drastically blocked oxygen; what your eyes need to stay fresh and healthy. Finally in the late 1960's there was break-through: Soft gel-like plastics called hydrogels. Hydrogels are soft and comfortable and they are hydrophilic (which means they can hold a lot of water).
That's key because the water in the lens material helps transmit more oxygen helping keep eyes healthier and feeling better. As a result, hydrogel contacts became the standard. Then, Johnson & Johnson Vision Care revolutionized hydrogel lenses by introducing the first disposables. And hydrogel lenses still improve millions of lives every day.
So, end of story? Not quite. Hydrogels work well but they still don't let your eyes breathe as easy as they'd like. High water content can mean a thicker lens for durability at the same time limiting the flow of oxygen.