Because a whole lot of work went into it. As I understand it, it was performed live in rehearsal and criticised and adjusted several times before being recorded.
The artists were spectacularly good, brilliantly cast, and seemed to 'become' their characters - this effect remains today. But mainly, It was 'clean '. No swear words - even the word 'bitch' which occurred once, was cut after the first showing. Any 'sex' angles were dealt with delicately and it was the dealing which was sensationally funny!
My favourite gag was when Joey went into hospital for a 'kidney stone'. Ross went with him, and when Joey did not understand what a Doctor said, he asked Ross, and Ross whispered in his ear. Joey was horrified, and said 'No way... tell them 'UP IS NOT AN OPTION!'
Sometimes the best humour is in what is NOT said!