Hi! In addition to the information Aaron provided, "Why did you come to -?" can sound very direct, which is the reason it's considered the less formal version. Said in the wrong tone, this can sound confrontational. "What brought you to-?" cannot sound confrontational because it's more formal. Opposite of what Aaron has observed though, I don't think that 'what brought you to-' is really less common. I use it all the time and I hear it frequently.

Of the two examples, "why did you come to-" is the more casual. "What brought you..." is a bit more old-fashioned and less common; it sounds more formal. I hope this helps!