Why not hijack the plane in India, or Bangladesh, or Pakistan, or Afghanistan, or Sri Lanka? All countries with plenty of jet airliners and millions of disaffected people with experience in terrorism.
Why hijack the plane in Malaysia a country half an ocean away?
People really need to engage their logic in this case, they are letting their imaginations run wild over what I suspect will eventually turn out to be a run-of-the-mill plane crash.
If one or more of the pilots are the hijackers, then that answers your question as to why KL was the origination point.
Otherwise, it may simply be that the terrorist cell they had available to pull this off was based in Malaysia. There was information given by a terrorist captured in Britain a few years that there was a Malaysian cell which included a pilot that was given a shoe bomb constructed by al Qaeda.
You say it’s “half an ocean” away. It’s not that far to India, it’s significantly shorter to India than it is to their original destination of Beijing.
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.