Why does \xspace behave differently for parenthesis vs. braces/brackets?

The author(s) of xspace didn't included ] and \} in the list of exceptions. Why? No Idea. But there is \xspaceaddexceptions which allows you to add them:

\documentclass{article}
\usepackage{xspace}
\xspaceaddexceptions{]\}}
\newcommand{\something}{something\xspace}
\begin{document}
(\something)

[\something]

\{\something\}
\end{document}

Result:

The xspace package uses a list of exceptions to decide when to add a space; the original definition of the list is:

\def\@xspace@exceptions@tlp{%
,.’/?;:!~-)\ \/\bgroup\egroup\@sptoken\space\@xobeysp
\footnote\footnotemark
}

As you see, ) is included in the list, but } and ] are not, so xspace will add a space. If you want to remove the space for this characters, add } and ] to the list by using \xspaceaddexceptions{]\}}.