Don't agree with that. Since no one was around to physically witness the evolution of any species, that would mean that our ideas on why any two species that shared a common ancestor evolved differently over time would just be generic statements. But in fact there are many such cases where one (by that I mean scientists, academicians) can make strong, educated guesses based on multiple factors like the fossil record, climatic differences, predators etc.

For example, the orangutans in Sumatra (especially the recently classified Tapanuli orangutan) are almost entirely arboreal whereas the Borneo orangutan is comparatively more comfortable on the ground. The educated understanding of the scientific community is that this likely happened because Sumatran orangutans evolved in close proximity to Sumatran tigers, unlike the Borneo species. That's not a generic answer and is certainly an interesting thing to ponder on.

That's the kind of thing I was looking for (to arrive at a probable/possible answer based on current evidence, if there is any). Not "tell me what the exact reason is, I want the exact reason!" I know you can't often find objective answers like that when it's evolutionary biology you're talking about.