The fashions which we now call "Standard dress attire" originated circa 1870-1910 in Britain, specifically London and vicinity. They are worn because they are accepted dress attire.
So long as it is modest, if something that resembles Jeans and a T-Shirt (maybe with elegant brocades or something, I hope, the concept fills me with dread) ever becomes "formal dress attire," then you can wear it to Mass. Until then, try not to wear something that you wouldn't be comfortable calling on Queen Elizabeth in. She's only an earthly monarch, Our Lord is your Heavenly monarch.
We are Catholics, and as such, have a respect for tradition. Tradition, the heritage of a Catholic-minded civilization, civilizes us. It tells us that it's NOT ok to go meet either monarch or God in T-shirt and jeans, or boxer shorts, or some kind of so overly-elaborate and out of place garment that one will attract undue attention.
One could, in fact, argue that the suit is unbecoming for another reason. The sack suit as we know it today originated as leisurewear. British gentlemen on their country estates preferred to hunt, rather than spend time at court. Consequently, the attire which we now consider a "suit" originated as a rather abbreviated hunting coat. The colors came from the industrial-revolution-age cities, where the dirt and soot made black, dark blue, and grey (the darker the better) ideal colors. Furthermore, the matching coat and trousers of a suit were originally a casual adaptation -it was considered more formal to wear trousers of a different color than coat or waistcoat. Presumably, the only reason that Traditional Catholic men "compromise" for this suit, which had its origins in leisure rather than formality, is that it would look out of place to wear any other, since the international canon of style has centered on the sack suit as the accepted dress in almost every public circumstance of note.
Also, it's worth noting that while some men, including myself, would love to go for the frock coat, there are several dissuading reasons. First, frock coats are exceptionally difficult to find today, given that they are never worn and would attract attention if they were. Secondly, frock coats have an unshakable affiliation, in our post-formal world, with protestant ministers who wore them as a sort of "garb of office" for many years, many still doing so. Finally, there's no need to go back to the frock coat, when the sack suit is both fully modest by all reasonable standards, and is the accepted daily dress wear.
One might ask why we don't wear tuxedos to Mass, given that they are more formal, and "after all, we must need to wear whatever we have that's most formal to Church, right?" In this case, however, the answer would be an unequivocal "no." Tuxedos are only to be worn in polite society after 5pm, or dusk, whichever occurs first. Therefore, as Mass is not usually after five, this explains the lack of black tie clothing. Further, even assuming Mass is after 5pm, Christmas Eve or Easter Vigil, for example, the general lack of such formalwear by most men prevents its appearance. Those men who do own it would be ill-advised to wear it, even should it be possible, given that it would at that point become conspicuous and distracting.
Distraction is only good if the principle demands it -this is why a man would be well within his rights to wear his suit at Mass, even if every other man were wearing a polo shirt and jeans. He ought to feel satisfied, knowing that he, at least, has upheld the decorum due at Mass, even if he is the only one who has done so. This is, in effect, why frock coats lingered as long as they did; the older men who wore them were unwilling to capitulate to the seeming loss of decorum, but the younger men recognized that the styles had simply changed. To illustrate this: I have a copy of Emily Post's superb book "Etiquette," from 1949. It states clearly that the sack suit is proper attire in public for all, and that the frock coat is equally proper, but only for "older men and clergy."
