Square brackets throw an error while parentheses don't

Question

Why does the following work

\documentclass[answers]{exam}
\begin{document}
    \begin{center}
        \Large\textbf{
            \shortstack{
                Assignment 1
                \ifprintanswers\\ (Answer)\fi
            }           
        }
    \end{center}
\end{document}

while this (after replacing the parenthesis with square brackets in the second printed line) doesn't?

\documentclass[answers]{exam}
\begin{document}
    \begin{center}
        \Large\textbf{
            \shortstack{
                Assignment 1
                \ifprintanswers\\ [Answer]\fi
            }           
        }
    \end{center}
\end{document}