The number one reason not to choose the BArch would be that you’re not entirely sure that you want to become an architect. it is not unusual for a high school senior with very little exposure to architecture to be in this position. The BArch programs require a strong commitment. They are intensely and narrowly focussed on architecture, 24/7. The drop out rate is high.
Other reasons to go for a BA or BS could include a wish for a more academically balanced undergraduate experience. A student might prefer the opportunity to explore other academic disciplines as an undergraduate. Or s/he may be drawn to a particular university/college that doesn’t offer the BArch.
The combination of the BA/BS + MArch is fairly common among architects, often with work experience in between degrees. The disadvantage would be that it takes longer and may cost more.
It also seems to me that many architects have both the BArch and the MArch, perhaps because they want to deepen their education, they want to supplement their focus from tech to design or vice versa or because they want a degree from a top rated university.
Neither path – the BArch nor the BA/BS+MArch – is better than the other: It’s really a personal decision.