Greetings Rabby,
Allah knows best, but here are some proposed reasons:
Prophethood is an office and Allah appoints a person to this office to fulfil a special need.
When the need arises for a Prophet, Allah appoints a Prophet to fulfil it. Allah does not send prophets in rapid succession when there is no need or when the need has been fulfilled. Some of the conditions under which Allah has appointed Prophets are:
-There was need for appointing a prophet because the message of an earlier Prophet had been forgotten by the people, or the teachings of the former prophets had been adulterated and hence it had become impossible to follow the message brought by that Prophet.
-The people had not received the complete mandate of Allah through a former prophet e.g maybe it was too much for them at one time. Hence succeeding prophets were sent to fulfil the task of completing the religion of Allah.
-There was need for a second prophet to share the responsibility of office with the first prophet.
None of the above needs remains to be fulfilled after the advent of Prophet Muhammad (peace be upon him).
The Holy Quran says that Prophet Muhammad (peace be upon him) has been sent for the whole mankind. Previous prophets had been sent to specific nations or groups of people. It follows, therefore, that different nations no longer need different prophets after the time of Prophet Muhammad (peace be upon him).
The Holy Quran and the records of Hadith and the biographical details of the life of Muhammad (peace be upon him) stand witness to the fact that the Divine message brought into this world by the Holy Prophet exists in its original and pure form. The Prophet's message has suffered no process of distortion or falsification. Not a single word has been added to, changed or deleted from from the Qur'an which the Prophet (peace be upon him) brought unto the world from Almighty Allah, nor can anyone make additions to or delete anything from it till the Day of Resurrection.
Also, the Holy Qur'an clearly affirms that God has finally completed His Divine Mission through the agency of Prophet Muhammad (peace be upon him). Hence there is no room for a new prophet to carry the divine mission to completion.
And lastly, if a partner were really needed he would have been appointed in the time of Prophet Muhammad (peace be upon him) to share the burden of his ministry. Since no co- prophet was appointed, this condition also stands fulfilled.
So when a prophet has been sent for the whole mankind and when the Divine message has been completely transmitted through this Prophet and further when the teachings of the Prophet have been fully preserved, Allah seals the office of prophethood after him in order that there may be unity in allegiance to this Prophet and one brotherhood of the faithful.
And Allah knows best.
Peace.
