The roast began in American comedy as a private club tradition and an occasional charity dinner. It is now a scheduled format with a dais, a running order and a marketing calendar.
The original version was closed and unrecorded
Early roasts were rooms full of professionals insulting one of their own, with no audience beyond the people present. The permission structure was mutual and local.
Because nothing left the room, material could be far harsher than anything broadcast, and the harshness was the affection.
Recording changed the incentives immediately. A joke aimed at one person in a ballroom becomes a joke aimed at everyone watching.
Television needed a repeatable shape
Broadcasters wanted the format annually, which required a predictable structure: a subject, a set of roasters, a closing rebuttal.
Predictability made the roast schedulable, and schedulable content is what a network can sell in advance.
It also fixed the length. Sets are timed and ordered so the strongest roasters are placed where the energy would otherwise sag.
The subject stopped needing a legacy
Traditionally the roast honored someone late in a long career. The televised version needs a subject with a current audience instead.
That produces subjects who are famous rather than accomplished, and the jokes shift from career retrospection to whatever is currently being said online.
The rebuttal set at the end has become the most important slot, because it is where the subject reclaims the evening.
Streaming made it live and unedited
Live streaming removed the edit, which historically softened the worst moments and rescued the sets that did not land.
Without an edit, a poorly judged joke stays in the record, and the audience reaction is part of the broadcast rather than something engineered afterwards.
That rawness is the appeal and the risk, and it is closer to the original club version than the television era was.
Writers do the work the dais gets credit for
Most roast sets are written with help, often by writers who never appear on camera, because the density required is far beyond a normal set.
A roast set is almost entirely punchlines, with the setups supplied by the audience's existing knowledge of the subject.
That density is why roast writing is treated as a specialty rather than as an extension of stand-up.