Why do other Muslims say that Ahmadis are non-Muslims?
Fundamentally in Islam, this right has not been given to any human being to declare others as non-Muslims. Religion is a personal matter and between the person and God and that is how the accountability is going to take place.
It appears that by declaring others as non-Muslims, such people want to create a sort of hatred in the minds of Muslims so that they are not interacted with. They think by declaring Ahmadi Muslims as non-Muslims they can be isolated and their message would not be heard. It is interesting to point out that Muslim clergy have not agreed on one definition of, who is a Muslim.