In one Question and Answer Session by Hadhrat Mirza Tahir Ahmad (ra), Khalifatul Masih the Fourth he pointed it out clearly:
My words: My recollection: (If any one can find the actual recording please let me know):
” Any community has a hierarchy. The Shias have their Ayatullahs, The Sunnis have their Mullahs or Sheikhs. Each has authority over his own community. All the different Jama’ats sort of accept the different hierarchies, as long as they remain with themselves. None of them claims universal authority and therefore they are not a danger for each other.
On the other hand the Ahmadiyya Muslim Jama’at, founded by the Imam Mahdi and Promised Messiah, who is followed by the Khalifas (and now headed by Hadhrat Mirza Masroor Ahmad, Khalifatul Masih V.) claim that all Muslims should now follow this ‘new’ Authority. Consequently all other Ayatullahs, Mullahs and Sheikhs feel threatened in their own Authority. Consequently they all have their opposition to Ahmadiyyat in common.
The opposition to Ahmadiyyat therefore is not really a religious thing. All of them, in one way or another, await for instance and Imam Mahdi or a return of a Messiah. We pray to the same Allah, we honor the same Prophet Mohammad, peace and blessings of Allah be on him, we say the same prayers, we read the same Qur’an. The religious arguments however are just the cover of the main challenge: The Ayatullahs, Mullahs and Sheikhs are afraid to loose their power and authority (and therefore also financing)… ”
Again: the above is my way of recollecting what Hazoor Khalifatul Masih IV tried to explain to us in a Question/Answer session. Any mistake will be mine… May Allah be our guide and protector always.