How is Lycra superior to rubber threads?

Materials aren't "good" or "bad" by themselves, you have to consider what you want them for as well.

Lycra is great for clothes where you want something stretchy, fast drying and maybe spiffy colors too.

Lycra absolutely sucks if you want something that's resistant to heat, flame and sparks.

Lycra is great if you want something that'll allow the skin to "breathe", but really sucky if you want something to keep the wind out.