1) Lack of institutional knowledge. No one even knows how to get started and bringing in foreign expertise may be prohibitively expensive.
2) Economics don’t pencil out even in higher income countries compared to BRT systems, especially because high density and heavy traffic means the lines usually have to be grade-separated which adds additional costs compared to an at-grade system.
3) Corruption makes development impossible. No well-established processes for expropriation exist, or the country is given over to clientelism such that landlords won’t give up what they own and hamper the development process via political connections.
BRT is usually the most effective solution in places where grade-separated rail is not yet viable as it allows a right-of-way network to be established that can later be upgraded to rail. This doesn’t solve problem 3, which requires a comparatively authoritarian approach to overcome the incentive problems at play; this is why the Chinese have generally excelled in the space over the last 20 years.