After the death of the Prophet (S), several Caliphs became leaders of the Muslim Ummah. I have two questions about this:
- Who was the last Caliph?
- Why is there no longer a set leader of the Muslim Ummah?
(This answer is according to Shia Islam)
A Caliph is appointed by Allah and no matter if people support him or not. Even if no one support and follow him he is still considered Caliph in view of Allah even if people do not consider him as Caliph. In fact the definition of Caliph is not who people select him. But it is who God select him and recognize him as Caliph. So according to Shia this Question is not true and already Caliph exist.
Who was the last Caliph?
Imam Mahdi S.A. and he is still.
Why is there no longer a set leader of the Muslim Ummah?
Main Leader is Mahdi S.A. and is already supporting and guiding Muslims but not publicly as all can see him (this does not mean no one can meet him). Prophet said: "Muslims benefit from support of Mahdi S.A. when he is in his hide time but do not see him. like people use the light of sun behind cloud." also Imam Mahdi had four representative who Muslims could contact him through them and Mahdi S.A. has set four required terms for the scholar who Muslims should follow him at his hide time and Shia scholars according to those terms agree one Mujtahid and appoint a qualified leader and currently he is Ayatollah Khamenei.