Because it's widely supported by emulators, and supports the 16 bit sample size at the 48 kHz sample rate that is needed for high quality music, which is more than enough for Tetris.
The SB16 also features FM synthesis, though this program doesn't appear to use it; it rolls its own wave table method.
However, that would be a particular reason to target it specifically, rather than some more modern card that lacks FM synthesis. FM synthesis produces the signature sounds heard in 80's pop music and video games.
non-PnP SB16 max out at 44.1KHz, only Vibra and later PnP models supported up to 48KHz.
The real reason is that SB16 is the lowest common denominator for DOS stereo compatibility with higher models. You might think SBPRo also supports stereo playback, but SB16, SB32, SB64, and SB128 do not support SBPro Stereo mode playback. SB16 DSP 4.x removed support for DSP 2.x 3.x "High Speed" modes. Anything using SBPro Stereo mode will not work in SB16 an newer at all.
The SB16 also features FM synthesis, though this program doesn't appear to use it; it rolls its own wave table method.
However, that would be a particular reason to target it specifically, rather than some more modern card that lacks FM synthesis. FM synthesis produces the signature sounds heard in 80's pop music and video games.