Here a very interesting piece on the sack suit history.
Excellent explanation and synthesis.
And very  interesting opinions about the sack:


"Traditional American Suiting: The Sack Suit:


At the turn of the 20th century, a distinctly American suit style emerged among the world’s fashions: the sack suit. Modeled after a French coat popularized during the 1840s, the sack suit was loosely-fitted, giving its wearer a soft silhouette. “Sack,” though, did not refer to the suit’s bagginess, but to a traditional French construction technique. Rather than forming the jacket’s back from four curved pieces of fabric, as was standard for formal wear, a ‘sacque’ coat was made using only two, straight panels. This technique gives the sack suit its characteristically ‘boxy’ look.

Traditionally, sack suits are:

    Single-vented
    Without shoulder-padding
    Without ‘darts,’ or folds that are sewn into a suit jacket’s canvas layer to increase the three-dimensionality of its elements, like lapels. Darts generally create a more ‘tailored’ look.

We owe the sack suit’s ascendance to American ubiquity almost solely to economic factors. Manufacturers were looking for low-cost garments to produce in large quantities, garments that lent themselves to industrialized production. Because the sack style was meant to look baggy, it was already a one-size-fits-all product: less variation was less expensive.

Established clothiers like Brooks Brothers and J. Press accepted this logic and set their machinery to full-steam. The style was quickly adopted as a collegiate ‘uniform’ by Ivy Leaguers seeking a care-free, but not entirely informal, style of dress. By the late 1950s, a sack-style suit was the standard for American business attire, despite it’s original French intention, extreme informality.
Contemporary Reputation

Today, the sack suit retains some of the “Ivy League” connotations with which it was born. Namely, a strain of American pragmatic conservatism. The sack suit has come to represent an “old guard” mentality, the States’ equivalent of a landed gentry.

But it’s lack of ‘fussiness,’ the fact that a sack suit can fit any body type and is generally cheap, ensures that it will never veer toward overt elitism. At one time, the sack style was an emblem of America’s democratic aspirations.
Decline In Popularity: A Speculative Interpretation

Still, the sack suit has fallen out of style in America. Why? There is the apparent aesthetic explanation: to contemporary tastes, a sack suit is ugly. There is a functional response: baggy suits don’t ‘work’ as well as those that fit. Then, there is the financial elitist’s response: because it’s mass-produced, it’s cheap; because it’s cheap, it bears no value for the wealthy. And the wealthy rule the world of fashion no less than any other.

I think that the sack suit’s decline in popularity can also be linked to America’s widespread villification of mass-production. Since at least the 1960s, mass-production has increasingly meant offshoring labor. Which is to say that, increasingly, mass-production is no longer America’s hope, but the doom of the American economy. Perhaps the sack suit’s impersonal silhouette bears a bit of America’s economic decline on its soft-shouldered back".

Stephen Hayward