The simple answer is that language tends to evolve, especially as it becomes more widespread. Linguistic theory is that Latin itself evolved from Proto-Indo-European. Latin as it’s own language went through several periods of development and evolution, and the Romance languages that replaced Latin did so because the language changed with the needs/times/technology of the people using it; language is a tool after all.

That’s the easiest answer as to why. As to how it changed, that is a far more complicated question, and one that I am not immediately able to succinctly answer. It’s definitely worth looking into though, especially because each separate language had its own distinct development, yet much of the grammar/syntax of Latin is still fairly prevalent. I can try to find a link to a useful Classics Department page that will shed more light on this aspect.