Why does the Event Horizon Telescope (EHT) not include telescopes from Africa, Asia or Australia?
Would it increase the diameter if they would include some from there?


No. Not by much, at least. The telescopes are already ~20,000 km apart, so you can't create a longer baseline that still has a simultaneous view of the target. 

Don't forget: Earth is a sphere. Only one half of that sphere can observe M87 at the same time. 

Telescopes in the Eastern hemisphere would enable more observations, but I don't know if that would bring any improvement over what they already had.