A- The first and the foremost reason would be the suffering and the affliction of humanity in general and the Muslim community in particular in accordance with the Text of the Qur'an:
"....va maa kunnaa mu'azzebin hattaa nab'ath rasoolan" Qur'an 17:15
Which translates to:
"We never afflict [i.e. a nation or a people. KH] with suffering/torment /punishment BEFORE sending a Messenger."
That is to say the Messenger must come first and then the suffering and afflictions will start. Muslim affliction started in 1260 A.H. after The Bab proclaimed His Mission, and this suffering is going to continue until it reaches it's apex until such a realization is born in every Muslim heart. The realization of this suffering will serve as a sign and a proof that Promised Christ of Islam has already returned.
Baha'u'llah reveals the following in regards to this final calamity:
"O YE PEOPLES OF THE WORLD! Know, verily, that an unforeseen calamity followeth you, and grievous retribution awaiteth you. Think not that which ye have committed hath been effaced in My sight. By My beauty! All your doings hath My pen graven with open characters upon tablets of chrysolite."-Baha'u'llah, The Hidden Words, Persian #63
Also: "The world is in travail, and its agitation waxeth day by day. Its face is turned towards waywardness and unbelief. Such shall be its plight that to disclose it now would not be meet and seemly. Its perversity will long continue. And when the appointed hour is come, there shall suddenly appear that which shall cause the limbs of mankind to quake. Then, and only then, will the Divine Standard be unfurled, and the Nightingale of Paradise warble its melody." -Gleanings from the Writings of Baha'u'llah, pp. 118-119
This calamity is in essence a SIGN from God which will inspire fear in hearts of the believers and serve as a catalyst in this process of transformation. The Holy Qur'an testifies to this issue:
"We refrain from sending signs only because the former generations treated them as false.... And We do not send the sign, except to inspire fear."-Qur'an 17:59
As the Arab idolaters had no option but to accept Muhammad, Muslims will have no option but to accept Baha'u'llah as the Promised Maseeh. This is the first point of similarity between Muhammad and Baha'u'llah.
B- The religious leaders are fast loosing ground and their followers are beginning to learn about the Baha'i Faith and its claims as well as basic elements of the Babi theology. Such was not possible during the lifetime of The Bab. Lack of education on a universal scale made it impossible for the average Muslim to self-investigate the claim of The Bab and decide for oneself. As a result Mahdi's claim was rejected by the population because of the directives of their religious leaders.
Science has become the nightmare of the religious leadership and education has opened the door to the questioning of traditional assumptions. Believers are starting to question the appearance of a Promised One on the clouds, falling of the stars, darkening/eclipse of the sun and the moon, raising of the dead, heaven and hell as actual locations etc... The absurdities instilled in the mind of the believers about the implications of these signs have been refuted by The Bab and Baha'u'llah in the most clear terms.
Baha'u'llah refuted the literalist understanding of these signs which the Gospels present as follows:
'Immediately after the oppression of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the earth shall be shaken: and then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory. And he shall send his angels with a great sound of a trumpet.' -Matthew 24:29-31
Similar prophecies such as the eclipse of the sun and moon and the descent of angels rank upon rank have also been foretold in the Qur'an which, for the sake of brevity, will not be cited here.