求助，textbf{}为什么只对纯英文有用，中文无效？

\documentclass[compress,xeCJK,note]{beamer}
\mode <presentation>
{ \usecolortheme{lily}
\usetheme{Madrid}
\usepackage{hyperref}
\setbeamercolor{normal text}{fg=black}
\setbeamercolor{alerted text}{fg=brown}%fg：前景，bg：背景
\setbeamertemplate{theorems}[numbered]
\setbeamertemplate{caption}[numbered]
}
\usepackage{wrapfig,graphicx,url}
\usepackage{amsmath,amsthm,bm}
\usepackage[utf8]{inputenc}
\usepackage{fontspec}
\usepackage{xeCJK}
\setCJKmainfont{SimSun}
\XeTeXlinebreaklocale "zh"%中文断行
\XeTeXlinebreakskip = 0pt plus 1pt minus 0.1pt
\begin{document}\small
\usebeamercolor{normal text}
。。。
\end{document}
这时导言区

的多维数组有:\[\begin{array}{l}
T = {x^{{k_1} \cdots {k_r}}}_{{l_1} \cdots {l_s}}{f_{{k_1}}} \otimes \cdots \otimes {f_{{k_r}}} \otimes {f^{{l_1*}}} \otimes \cdots \otimes {f^{{l_s*}}}\\
= {y^{{i_1} \cdots {i_r}}}_{{j_1} \cdots {j_s}}{{\tilde f}_{{i_1}}} \otimes \cdots \otimes {{\tilde f}_{{i_r}}} \otimes {{\tilde f}^{{j_1*}}} \otimes \cdots \otimes {{\tilde f}^{{j_s*}}}
\end{array}\]
就这一段