I suppose because "handsome" is normally used to describe men and masculine things... but dictionary.com disagrees: http://dictionary.reference.com/browse/handsome?s=t
That is the widespread usage of the word, but it is not the English language that prohibits using it for non-man things. As you know English does not have grammatical gender for adjectives so "handsome" cannot be masculine, it is only a word carrying a meaning of some kind of beauty. And since most English speakers, for cultural reasons which I disagree with, usually do not describe a man with words of extreme beauty nor delicateness even if he is so qualified, they tend to use words showing moderate beauty like "handsome" and "good-looking" and they tend also to ignore the even more general word "beautiful" which can describe every single thing, you can say beautiful child, beautiful man, beautiful woman, beautiful car, beautiful sky, beautiful tree, beautiful animal, beautiful idea, beautiful voice, beautiful language, beautiful work, beautiful formula, ... So they have to exclude the man from all this because they feel the word is irrelevant when it comes to men.
Probably because beautiful to the American mind carries with it the idea of elegant -and most American men view elegance as a delicate and feminine quality. Right or wrong, most men don't want to be seen as elegant, delicate, or beautiful we typically wish to be viewed as rough, rugged, strong and durable able to protect and defend those who are beautiful, elegant, and delicate
For better or worse this has been changing in the past few decades, and yet most American men still feel uncomfortable being referred to as pretty or beautiful. (Including myself)
"Bello" is masculine, where "bella" is feminine. You would use "bello" to describe a masculine word, such as "uovo". You would use "bella" to describe a feminine word, such as "tazza". You can tell whether a word is feminine or maculine based on if it uses "la" or "il". In this case, you would use "bello" because it says "il vestito", indicating that "vestito" is a maculine word.
Well, yes. That's what the word "looks" means in that sentence. It does not mean that the dress has eyes and is looking at something that's nice. The word "looks" has the sense of "seems" here. Therefore, both "The dress looks nice" and "The dress seems nice" would translate to «Il vestito è bello.» This is my constatation. I hope a native Italian and English speaker could verify. «It seems to be nice» I believe would «Sembra sia bello.»