Ed Glaeser:
Another area of agreement among transportation economists is a profound enthusiasm for buses over trains. Bus rapid transit is considered a very high-return investment. These aren’t necessarily buses operating on crowded city streets; these are buses with dedicated lanes that can achieve almost the same speed as trains.
The beauty of buses, from a cost-benefit perspective, is you don’t need to lay down massive infrastructure that you’re stuck with forever. If a bus route doesn’t attract enough people, you switch the route. Or you stop running it. It’s flexible in a way that trains aren’t. And that’s tremendously valuable in a world of uncertainty.
But there are very real reasons that we have a hard time building BRT in his country. First, it isn't actually cheap to carve out dedicated lanes in existing densely populated areas. Those areas don't always have wide streets from which you can easily take a few lanes. And once you add all the bells and whistles (boarding platforms, offboard payment, signal priority,etc...) that really make BRT BRT, you don't end up saving much money.
Then there's the praised flexibility of BRT. A big problem is that flexibility exists in the design and build stage, when it's easy to keep cutting corners, and therefore costs, until you're just left with a bus. Costs aside, the idea of using dedicated lanes sounds great until you do battle with the local drivers, who would rather hand over their mothers than give up a car lane for those bus people. Those dedicated lanes gotta come from somewhere, and in a dense area that justifies the expense of BRT (we only build roads to nowhere, not transit), the existing traffic is also dense and the existing drivers an entrenched interest.
All for BRT and in general more and better buses, but it isn't that simple...
tl;dr BRT isn't that cheap, it's too easy to snip costs everywhere until you're just left with B, and political opposition to using dedicated lanes is huge.