There are two major reasons.
- Primarily because Kerala has a monsoon climate and because of the heavy rains, the roofs are gabled so that water doesn’t stay on the roof. The houses that were built in the 90’s when concrete roofs were the trend, tried to do away with this gabled design. However, soon it was found that water which stayed on the roof began oozing into the walls and also weakened the structure. Now the trend is again to go back to the gabled design. This same pattern can be found in most of western India where rains are very strong. In places of heavy snowfall, like in many places of Himachal Pradesh, Kashmir etc. also you find this pyramid shaped roof.
- Olden Kerala roofs were either made of thatched leaves or wooden framework covered by tiles. Therefore a pyramidal structure could support more weight rather than a flat design. A flat design would need stronger pillars.