Comparison between fire tube boiler and water tube boiler?
The reason is to be found in thermodynamic nature. To get a high efficient water-steam-cycle a high temperature and pressure of the steam is required. The higher pressure and temperature the more effective process you get.
Water tube boilers are better suited for the high pressures and temperatures requirements. Higher pressures require greater wall thickness, but this reduces heat transfer. The practical limit is about 16bar (240psi).
Fire / smoke tube boilers are also more prone to catastrophic failure. The water - and energy - in a Scotch boiler, say at a modest pressure of 10 bar, is enough to launch the boiler through bulkheads and accident in the boiler room.
Water tube boilers lend themselves to modern modular construction techniques more than fire tube boilers. Even in field-erected water-tube boilers, most of the fabrication - for instance, water-wall panels - can be done in a workshop under controlled conditions or using automated equipment. This improves quality and reduces construction time and costs.
Fire tube boilers are due to their big shell diameter not suitable for high pressure and temperature.
A fire tube boiler would have to be very large, and all that energy in the water is just sitting there. If there were a long seam burst, the deluge of water and steam would be immense and could injure many people. Also, due to its mass, and fact that they are floor mounted, they create a lot of pressure on the foundation. So, the nature of most fire tube boilers is for small to medium manufacturing facilities. Many plants have 2 or more and these can be distributed around the plant where needed.