The answer usually lies in 19 century politics.
It was too costly I think in a century without phones and internet to keep such territory under control.
At least that was the reason the great colombia failed, because back then people didnt like to be controlled by far away regions.
Venezuelas separated because they didn't want to be under bogota rule.
Most of the time, diferent nations have diferent goals that made them to be separated.
>But they're the same people
Kinda, but it would be like saying that hokkaido and ryushu islands are the same as tokio fags.
Generally in latinoamerica there's like 5 diferent sub regions, so at least hispanoamerica would need to be 5 diferent nations (mexico,caribean, great colombia, brazil, peru+bolivia, southern cone).