It’s the current style. Style’s change over time.
Not all frum Jews wear black hats. Many people prefer a fur hat, and some prefer just a yarmulke. Of those wearing black hats, fedoras are currently the most popular though it is not uncommon to see other styles (homburgs).
Jews have a tradition of dressing up for Shabbos, which probably goes back to ancient times. American goyim had similar customs of wearing their best clothes on Sunday until the mid-20th century, and some still dress up (African Americans in particular). However we dress up in our “interview suits” (well, HaShem is the Boss, isn’t he?).
Jews have always prefered modest and solemn styles, so even in centuries where men wore highly colorful outfits and women wore outfits designed to show up the parts that are normally covered, Jews tended to prefer more somber clothes. This in part reflects mourning for the destruction of Jerusalem.
It’s interesting the frum kids trying to being “rebels” currently dress up in colorful shirts without jackets and ties (the “in style” of 20 years ago), whereas the “cool” goyim are now preferring suits and ties, often with white shirts, and some are beginning to wear hats again. However, in general we tend to be late in imitating goyish styles. Note the late adapation of long pants by frum Jews, and the survival of fur hats, or how double-breasted suits survived longer among us than among the goyim — often we haven’t given up a style that the goyim long gave up on, and it comes back among the goyim.