Because vulgarity is degrading, and enhances a conversion about much as passing gas does in close company, and both bring attention to the source. Meanwhile, I have enough issues with my own heart without being reminded of a bedroom and or bathroom by listening to someone.
If our speech and or writing was a plate of dinner, would anyone want to eat it?
Moreover, profanity exposes the destitute authority of the speaker, as he must invoke deity for authority.
Thou shalt not take the name of the Lord thy God in vain; for the Lord will not hold him guiltless that taketh his name in vain. (Exodus 20:7)
Let no corrupt communication proceed out of your mouth, but that which is good to the use of edifying, that it may minister grace unto the hearers. (Ephesians 4:29)
But shun profane and vain babblings: for they will increase unto more ungodliness. (2 Timothy 2:16)
Ruggum Smuggum always works