Bitcoin has an image problem. Most people on the planet see it as something "weird", "underground", "unofficial", "dangerous". Lack of knowledge for sure, but still. The fact that a country officially adopts it is reassuring (even if it appears as a paradox for bitcoiners, since Bitcoin represents an "anti-government" stance).

Becoming legal tender in various places would enforce the idea that Bitcoin is a de facto real currency. This is why all institutions from the old fiat world (IMF, ECB and such) tried to ridicule Salvador and are still fighting so much, even rejecting/banning the use of the word "cryptocurrency". They will do everything they can to avoid that Bitcoin reaches "real currency status". And the only way to fight back is to make Bitcoin more "official".