There is no doubt that in comparison with red bricks, fly ash bricks are structurally stronger, more durable, better insulators, water-resistant, etc. But not using them so usually has, especially in India, a whole lot of reasons :
Firstly, though fly ash bricks save the cost of the project as a whole, they are still more expensive than red bricks, and the poor majority doesn’t like the idea. Secondly, flay ash bricks are not easily available everywhere, and red bricks are. Getting fly ash bricks transported adds to their cost, which already was higher.
Thirdly, only modular sizes are feasible. Larger bricks are very brittle. The outer surface is harder as compared to the inner core, which can lead to cracking in the future if a few bricks are cut out to allow running of pipes, wiring, etc.
Fourthly, laborers aren’t very familiar here with fly ash. Many builders, therefore, consider it a bad choice.
Fifthly, fly ash bricks buildings get quite cold in winter since it doesn’t absorb much heat.
Sixthly, fly ash bricks are only good if care for quality in production has been taken.
Seventhly, they require cement to be produced, increasing the demand for cement. Cement production, in turn, is not environment-friendly.
Lastly, fly ash is a health hazard. It has got substances that can reach in and stick in your lungs, causing long term damage. Crystalline silica, a component in fly ash, is a known carcinogen.