Question:

If Moshiach did not come in earlier generations, in spite of the greatness of the people who lived in the past, why should we expect him to come in our generation? 

Answer:

Our generation possesses the cumulative good deeds of our forbearers, with the addition of own accomplishments. 

Although it is true that each generation is spiritually regressive compared to the one before it, this fact in no way repudiates Moshiach's eventual arrival.  We know that Moshiach must arrive within a certain time period.  The more time elapses, the more certain we are that Moshiach's coming must be imminent.

Exile is compared to pregnancy, and the coming of Moshiach has been likened to the birth of a child.  The more advanced the pregnancy, the closer is the birth of the child.  Similarly, the fact that we have been in exile for so long indicates that Redemption must be at hand.

Although we may be spiritually inferior to our predecessors, our generation is like a dwarf standing on the shoulders of a giant, who can see further than the giant himself.  We possess the cumulative good deeds of our forbearers, with the addition of our own accomplishments, however modest.  In fact, in certain respects we are spiritually more advanced than previous generations, as we have had to overcome greater challenges.

The signs mentioned by our sages as indications of Redemption have never been more in evidence than in our generation.  The Lubavitcher Rebbe has declared repeatedly - also invoking the role of prophecy - that the Redemption will come imminently in our generation.  As the Rebbe himself stressed, these words were based on the fact that we have accomplished all that is required of us in refining the world.  All that is left are some minor things, including recognizing the signs of Redemption and accepting Moshiach's leadership upon ourselves.