Why did South American navies move beyond coastal-defence ships?

Would South American navies have been better served by coastal-defence ships instead of battleships?

Only for a time; their long coastlines and dispersed ports increasingly required the speed and range of full battleships.

A slow local ship can defend a compact archipelago or a small number of harbors, but covering Brazil's or Argentina's coastline would require many such vessels. As South American economies and port networks expanded, navies still wanted the armor and firepower of coastal ships while also needing rapid movement between distant areas. That combination favored full battleships and eventually dreadnoughts.