The 13th, 14, and 15th amendments were specifically written for the special case of newly frees slaves.

The 14th amendment was ratified in 1868. The total population of the US in 1870 was around 38 million. The total number of immigrants was around 2 million. The slave population was around 5 million.

The implications of the 14th amendment are clear today and they may even have been clear then. Nevertheless the most pressing, most politically salient issue was the newly freed slaves. In one day (the day after ratification) the United stares gained 5 million new citizens.

I would not be surprised at all if the people involved at what time were a tad bit myopic, with respect to the problem at hand.