Why aren't my equations "pretty" (italicized)?

Question

I'm making a presentation in LaTeX using beamer. I have typed up the Navier-Stokes equations, but for some reason many of the variables (the superscripts and subscripts, Y, T, x, etc.) are not italicized. Why might this be happening?

Example:

\documentclass{beamer}

\usepackage{graphicx}
\usepackage{amsmath}
\usepackage{xmpmulti}

\begin{document}
\title{\LARGE title}
\author{author}

\frame{\titlepage} 

\frame{\frametitle{Table of contents}\tableofcontents} 

\section{Section} 
\subsection{Navier-Stokes Equations}
\frame{\frametitle{Navier-Stokes Equations}
    \begin{itemize}
        \item The compressible, reacting Navier-Stokes equations for an ideal gas can be written as
    \end{itemize}
    \begin{align*}
    \frac{\partial \rho^d}{\partial t^d} + \frac{\partial\rho^d u_{j}^d}{\partial x_{j}^d} 
    &= 0,\\
    \frac{\partial{\rho^d Y^d_k}}{\partial t^d} + \frac{\partial{\rho Y^d_k u^d_j}}{\partial x^d_j} 
    &= \frac{\partial}{\partial x^d_j}\left(\rho^d D^d_k \frac{\partial Y^d_k}{\partial x^d_j} \right)+\dot{\omega}^d_k,\\
    \frac{\partial \rho^d u_{i}^d}{\partial t^d} + \frac{\partial\rho^d u_{i}^du_{j}^d}{\partial x_{j}^d}
    &= -\frac{\partial p^d}{\partial x_{i}^d} + \frac{\partial \tau_{ij}^d}{\partial x_{j}^d}, \\
    \frac{\partial \rho^d E^d}{\partial t^d} + \frac{\partial (\rho^d E^d+p^d) u_{j}^d}{\partial x_{j}^d}
    &= \frac{\partial\tau_{ij}^du_{i}^d}{\partial x_{j}^d} + \frac{\partial}{\partial x^d_j}\left(\mu^d\frac{c^d_p}{Pr}\frac{\partial T^d}{\partial x^d_j}\right)+ \sum\limits_{k=1}^{N}Q^d_k\dot{\omega}^d_k \\ 
    p^d &= \rho^dR^dT^d = \rho^d \frac{R_u}{W^d}T^d
    \end{align*}
}

 \end{document}