HD stands for High Definition, in other words, way bigger resolutions than the 640x480 the original game was made for back in 1998.
The loss of the original artwork prevents the devs from re-rendering (exporting for use in the game) said artwork in HD, or, the bigger resolutions we have available nowadays.
Thinking about it.. wouldn't it be possible to add some filters on top of the game to smooth out the low quality original graphics in higher resolutions? They did similar stuff in GTA3 and San Andreas mods to improve graphic quality.
If you've ever played a SNES game on an emulator with video interpolation you'll see the limitations of such technology. In many cases it can make the original art look worse.
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The loss of the original artwork prevents the devs from re-rendering (exporting for use in the game) said artwork in HD, or, the bigger resolutions we have available nowadays.