WHY ONLY RAM RAJYA AND NO KRISHNA RAJYA?

This question can only be answered in the context of History, after accepting of course, that Shri Krishna and Shri Ram were incarnation of Lord Vishnu. 
Only in the background of history the question is pertinent; Shri Krishna, as avatar was born with 16 Kalas yet he could not provide a certificate to future generations that he was totally and comprehensively successful, whereas Shri Ram was born with 14 Kalas yet Ramrajya provides a certificate to future humanity that Shri Ram was totally and comprehensively successful. 

It is important to understand that as per Sanatan Dharm the maximum Kalas with which an avatar can come on this earth is 16.

Yes there was no Krishnrajya, and in the context of history, this does mean that Shri Krishna was not comprehensively successful, but Shri Ram was. WHY?

We will move with simple answers first. 

1. The scientific development preceding Ramrajya was far less than the scientific development that confronted Shri Krishna. The rate of scientific development too was much slower, as during Treta Yug things take shape SLOWLY as compared to Dwaper Yug. So it was easier for Shri Ram to bring in complete and comprehensive changes.
2. The other important thing which even Avatar cannot resolve or manipulate is the cyclic theory of evolution. Mother earth is almost fully charged during Treta Yug, whereas mother earth has already utilized 9 out of 10 periods of onslaught on its resources at the end of Dwaper Yug, and is looking forward to recharging at the end of Kalyug, the last of the 10 periods.

Conclusion: nature is exhausted, needs recharging and is shrinking when Shri Krishna arrives on earth. Read: MODERN SCIENCE NEEDS TO UNDERSTAND CONCEPT OF KAMDEV IN SANATAN Dharm

Ok, now let us understand that apart from the correction of direction in which humanity is moving, Avatar, in order to be successful comprehensively, would be required to usher in physical parameters for all inclusive growth of entire civilized society, and nature. Remember, Sanatan Dharm is followed by Hindus but Sanatan Dharm does not recognize boundaries created by followers of different religion. 

So Ramrajya means all inclusive growth of entire humanity, plus nature, with no room for exploitation by way of caste and creed. That also means after Ravan’s death, and after Sita’s disownment by Shri Ram, the exploitation lobby of Brahmins ended. 

Shri Ram was successful in ABOLISHING CASTE SYSTEM, which was prevailing in the Hindu Society right from the start of Satyug. We also know from the history that we have in Puraans that this abolishment of caste system did not last for a long period and opportunists and rich and wealthy in the society were able to re-establish the caste system again. 

Repeat: Avatars usher in positive changes, arrest rot, but this does not last long. The rich, powerful plus religious leaders are too strong and manipulative to resist. World history repeatedly confirms this.

Post Mahabharat, the clues of the history of the world immediately after Ashwasthama’s chemical device to destroy or affect all future pregnancies did cause panic and caused wide spread movement of already fragmented societies. Even Dwarika witnessed large scale arson and looting. 

The entire civil and social structure after such a massive war collapsed. There was no room for Krishnrajya. The issue was of SURVIVAL for remaining humanity after Mahabharat. KRISHNRAJYA COULD NOT EVEN BE CONCEIVED.