For the bathroom taps, I think the reason is purely technical. There are two pipes, one for cold water, one for hot water, each pipe gets its tap. It is the simplest setup and with that, you can control both flow and temperature. Systems that have a cold-hot knob are slightly more complex and therefore they are more expensive and less reliable, especially when they are thermostat controlled. And with a single, one axis knob, you can't control both flow and temperature.

I don't know how it works in India, but I know some places where there is no centralized hot water and only a single pipe. When you need hot water, there is a dedicated heater and you control temperature on the heater itself. So I guess that someone who is used to a system like this will find a two tap system unsettling.

Anyways, I think we can all agree that the worst system is the one with two separate taps with no mixing, like they still have in the UK.