Paintings, photographs and various other historical sources all show that even in the late 19th century, people typically wore folk costumes, be it Lederhosen in Bavaria, the Fustanella and fez in Greece or the Kroj of Slovakia. In some parts of the world, traditional costumes are still an every-day sighting, alongside modern, global outfits - such as the Sari in India and the Deel in Mongolia, or the Chullo in Peru.
My question is: why did these beautiful (I know, there's a sentimental bias there) and cultural outfits largely die out, pretty much entirely in Europe with the exception of folk dancers or festivals/parades, in favour of global clothing? When exactly did this occur? Whilst I'm sure it was gradual, is there a date we can say was the catalyst of the downfall of national costumes?
On a side note, why did people stop wearing hats? Almost everyone in the past seemed to wear hats, whether they were regularly outdoors or not.
Not to discourage future answers, but the FAQ has an extensive section on the history of fashion, including the supposed universality of suits and ties as business wear.
Similarly, for your last question, if you search the /r/AskHistorians archive for questions about hats, quite a few results show up.
I would ask to what extent the 'folk costume' is itself a traditional garment and to what extent it is a 19th century nationalist statement. We might also ask when national costume came to be adopted, for what purpose, and why.
Edit: Come to think of it, I will make this its own question.
I'm less knowledgeable about t-shirts, but for suits in The Birth of the Modern World, Bayly argues that the trend towards uniformity was brought about by fashion and advertising, and an increasing perception that to be taken seriously it was necessary to dress by Western standards. There was a large reaction against this, and many, both non-Westerners and Westerners, considered it a grotesque parody, and an unnecessary decision (A few Western observers believed African and Indian politicians should not wear shoes). It was a slow process, and normally seen in the 'toning down' of national dress, slowly dropping more and more parts of the traditional garb in favour of a more sombre and 'serious' outfit. Interestingly he also mentions the poor, and that they barely changed their method of dressing throughout the 19th century, continually wearing cast-offs, patched garments and a muddle of different styles and eras. We should not make the mistake of thinking that national dress, such as the fez or lederhosen, were widely worn, as they were often too expensive for the majority of the population to wear.