Because its orginal intention (see http://www.joelonsoftware.com/articles/Wrong.html and http://fplanque.net/Blog/devblog/2005/05/11/hungarian_notation_on_steroids) has been misunderstood and it has been (ab)used to help people remember what type a variable is when the language they use is not strongly typed. In any strongly typed language you do not need the added balast of prefixes to tell you what type a variable is. In many untyped script languages it can help, but it has often been abused to the point of becoming totally unwieldy. Unfortunately, instead of going back to the original intent of Hungarian notation, people have just made it into one of those "evil" things you should avoid.

Hungarian notation in short was intended to prefix variables with some semantics. For example if you have screen coordinates (left, top, right, bottom), you would prefix variables with absolute screen positions with "as" and variables with positions relative to a window with "rel". That way it would be obvious to any reader when you passed a relative coordinate to a method requiring absolute positions.

*update (in response to comment by delnan)*

IMHO the abused version is avoided like the plaque because

- it complicates naming. When (ab)using Hungarian notation there will always be discussions on how specific the prefixes need to be. For examply: listboxXYZ or MyParticularFlavourListBoxXYZ.
- it makes variable names longer without aiding the understanding of what the variable is for.
- it sort of defeats the object of the exercise when in order to avoid long prefixes these get shortened to abbreviations and you need a dictionary to know what each abbreviation means. Is an "ui" an unsigned integer or an unreferenced counted interface? And those things can get long. I have seen prefixes of more than 15 seemingly random characters that are supposed to convey the exact type of the var but really only mystify.
- it gets out of date fast. When you change the type of a variable people invariably ( lol ) forget to update the prefix to reflect the change, or deliberately don't update it because that would trigger code changes everywhere the var is used...
- it complicates talking about code because as "@g ." said: Variable names with hungarian notation are typically difficult to pronounce alphabet soup which inhibits readability and discussing code because you can't 'say' any of the names.
- ... plenty more that I can't recall at the moment. Maybe because I have had the pleasure of not having to deal with the abused Hungarian notation for a long while...