Because India has no Communist ties, right?
I don't think it's a huge stretch to suggest that Nepal's proximity to Red China helped the takeover. Mind the ironic imagery.
But yeah, pretty slow domino.
Ok, let's be serious here for a second.
1) The Indians only got involved with the Reds (like Nassar) because they offered a better perk package.
2) How do you explain the Sandanistas? Sure, proximity makes agitation easier / puts like-minds together, but so does a pissed-off and fed-up populace. It's only one piece of the puzzle.
3) Domino theory never worked out predictably anywhere and probably never will. You're either pre-disposed to Scarlet Fever, or you aren't.