Why the dispatchers didn't reroute the flight plan, I mean ..they should be used to that kind of weather in that area.
Re-route where?
I'm no expert, I'm not a local, but I have flown across that part of the world more than once or twice going into/out of SIN. It's the tropics - it's pretty much a given that the sigmet chart will show a rather large area of Cbs/Turbulence/hail/dragons
somewhere across your route, in fact chances are the forecasters have covered their collective ***** and depicted it over most of Indonesia, Malaysia and quite possibly over your departure/destination airport....so how are you going to design a re-route at the planning stage?
What's wrong with using the weather radar and making sure you carry the fuel to be flexible? The dispatcher (if your airline uses them) and/or the flight plan can't provide the solution to everything...I think the answer has to lie on the Flight Deck and the word I'm looking for is "Airmanship" (yeah, I'm that old..).