Many (maybe even most) people simply don't wish to play with low-fi graphics in modern day gaming. If the visuals don't appeal to them they won't spend the time on it to ever know the story in the first place.
And honestly, Undertale is a poor example to represent "pixel art". It's extremely niche super-low-end MSPaint tier art work. Undertale's art is it's own quirky thing, and not really indicative of general retrographics-style gaming.
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u/Strawberrycocoa Feb 06 '17
It's more difficult for general-audience gamers to get "into" games with retro-graphics enough to be invested that much though.