El Salvador is a small, very poor country (average monthly income = $365). And they are a democracy. If you are an average citizen of El Salvador, you are probably thinking about the nice things you could get if your government sold all of that BTC at a huge profit (and yes, math: this isn't enough to make everybody there rich or something, but again, this country is a democracy, so...).
I know El Salvador doesn't have a large percentage of the total Bitcoin supply, and them dumping it would only be a small bump on the market for a day or two, but I would reckon the psychological effects would be pretty huge.
Of course you could argue that they would want to keep their BTC as an investment, given how much money they have already made on it, but why not invest in some other digital currency that has potential gains of 10x more? In other words, what if El Salvador is gripped by... mass-FOMO .
Something to think about...