Because CRISPR isn’t nearly that precise or exact, it always has unforeseen random fall off effects.
Also, who says that particular receptor ONLY prevents entry of a specific virus? It surely has other purposes that aren’t understood.
https://molecularsciences.org/content/the-challenges-of-cris...
I've yet to see anyone give anything but unknowns as their answer for why it's bad, or what might happen. Nobody seems to have any specific pathway for bad things to happen in mind.