I think most go mumble because it is free.
Im sorry im new, waiting until 1.20 to make my new character, but was wondering myself about this and just wondering, how exactly is mumble free?

The cheapest server i found that they offer is a 10 slot one for one month and that costs $4

unless if you are talking about a public 8 man server, those are free, but team speak and vent offer those for free as well