Malaysia Air is notorious for sloppy security.
There are many non-Muslim Chinese in Malaysia, especially on a flight to China that few Muslims go to.
The other countries you name are Muslim and flights to and from them are going to be full of Muslims.
You’re not going to get your 43 virgins if you kill a plane full of Muslims.
There were dozens of Muslims on board the flight including the pilots, cabin crew and a heck of a lot of passengers. There would be much fewer Muslims in a plane flying from Sri Lanka or Southern India and those places are one heck of a bit closer to, you know, India than Malaysia is.