Is there something about Isaiah that separates him from Israel as he confesses his sin? He confesses that he is unclean, as unclean as his fellow countrymen (Isaiah 6:5) but then he is cleansed and he alone is set apart. He will alone be set apart to go and speak to Judah.
It appears that the same grace was available to Israel as a whole. It was offered by God through Isaiah in 1;18. It appears that their pride kept the nation away from such an offer and the LORD sees fit to cut them off because of that unconfessed sin.
Something happens with our fellowship with God when we sin. But something else, something deeper, seems to happen when we are confronted with it by God and then refuse to confess and repent.