The reason for this is unknown.

Theory among the younger members traces it to the disgraceful price of oysters; it is more probably due to a desire to come to the point, to a good practical sense deciding at once that hors d'oeuvre are but poor things.
The Jameses alone, unable to withstand a custom almost universal in Park Lane, are now and then unfaithful.
A silent, almost morose, inattention to each other succeeds to the subsidence into their seats, lasting till well into the first entree, but interspersed with remarks such as, "Tom's bad again; I can't tell what's the matter with him!" "I suppose Ann doesn't come down in the mornings ?"--"What's the name of your doctor, Fanny ?" "Stubbs ?" "He's a quack!"-- "Winifred?
She's got too many children.

Four, isn't it?
She's as thin as a lath!"-- "What d'you give for this sherry, Swithin?
Too dry for me!" With the second glass of champagne, a kind of hum makes itself heard, which, when divested of casual accessories and resolved into its primal element, is found to be James telling a story, and this goes on for a long time, encroaching sometimes even upon what must universally be recognised as the crowning point of a Forsyte feast--'the saddle of mutton.' No Forsyte has given a dinner without providing a saddle of mutton.
There is something in its succulent solidity which makes it suitable to people 'of a certain position.' It is nourishing and tasty; the sort of thing a man remembers eating.

It has a past and a future, like a deposit paid into a bank; and it is something that can be argued about.
Each branch of the family tenaciously held to a particular locality--old Jolyon swearing by Dartmoor, James by Welsh, Swithin by Southdown, Nicholas maintaining that people might sneer, but there was nothing like New Zealand! As for Roger, the 'original' of the brothers, he had been obliged to invent a locality of his own, and with an ingenuity worthy of a man who had devised a new profession for his sons, he had discovered a shop where they sold German; on being remonstrated with, he had proved his point by producing a butcher's bill, which showed that he paid more than any of the others.

It was on this occasion that old Jolyon, turning to June, had said in one of his bursts of philosophy: "You may depend upon it, they're a cranky lot, the Forsytes--and you'll find it out, as you grow older!" Timothy alone held apart, for though he ate saddle of mutton heartily, he was, he said, afraid of it.
To anyone interested psychologically in Forsytes, this great saddle-of-mutton trait is of prime importance; not only does it illustrate their tenacity, both collectively and as individuals, but it marks them as belonging in fibre and instincts to that great class which believes in nourishment and flavour, and yields to no sentimental craving for beauty.
Younger members of the family indeed would have done without a joint altogether, preferring guinea-fowl, or lobster salad--something which appealed to the imagination, and had less nourishment--but these were females; or, if not, had been corrupted by their wives, or by mothers, who having been forced to eat saddle of mutton throughout their married lives, had passed a secret hostility towards it into the fibre of their sons.
The great saddle-of-mutton controversy at an end, a Tewkesbury ham commenced, together with the least touch of West Indian--Swithin was so long over this course that he caused a block in the progress of the dinner.