Xbox emulation is notoriously difficult. The Nvidia NV2A GPU (a modified GeForce 3) and the Pentium III CPU are easy enough to emulate, but the I/O architecture is weird.
A standard ISO file copies an entire disc, including the empty space and security padding. Microsoft used a proprietary file system called for its game discs. Xbox Xiso Roms
That dynamic has shifted drastically in the last five years. Two factors have driven the resurgence of the XISO: Xbox emulation is notoriously difficult