Increased mass.
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Scott Wallace
Bass strings are wound to make them heavier but still remain elastic. Plain metal (or nylon, or gut) strings don’t sound good in the bass.
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Brian Coverstone
There are three things that change the pitch of the string. The string thickness, the length, and the tightness. When you tune your guitar, the only thing you are changing is how tight the string is pulled. When you set your intonation, you are changing the length (slightly). The thickness of the string is static and cannot be changed, unless you buy different gauge strings.
Given that, bass strings must be a lower frequency. So they must either be very long, or very thick. Since we can’t have 6 foot long bass strings, we have to make them thicker. But if you make a string too thick, you’ll end up with a solid metal rod that won’t vibrate much. So to compensate, they take a thin string and wind copper or nickel around it to make it thicker. That technique allows a string to remain flexible and vibrate.
Try taking your high E string and tuning it to the low E frequency on your tuner. You’ll begin to see why it must be thicker.
https://www.youtube.com/watch?v=8tfxy_zs0Mo&list=TL9CIQinYQUXnHjd3x5NothVrL5QK8DFG3
