Why dose Paraguay have more Guarani speakers than Spanish speakers?

Now I know countries like Guatamala have more natve lsnguage speakers than Spanish speakers. But Spanish still has the most speakers of any language spoken there.

But Paraguay is the only know were a natibe language has more speakers than a european language. Its the only one were the non natives learnt the native language. Like how many non Miztecs can speak Nahutal? I'll give you that many white South Africans speak Xhousa and I think most Rhodasians speak Shona.

What made Paraguay the exception? Was Gauarni the only language in Paragauy prior to colonialism? Ive read that during their war with everyone else thryd only speak Guarani so the Brazilians couldnt understand them. Or that most of the Spanish population was killed or a much bigger % of Spanish Paraguyans were killed than Guarani Paraguyans.

Why is Paragauy the outlier?