Why did it happen that in the Ukraine, there were so many tsaddikim? Is there any comments on that by wise men of judaism?

Since our exile from Israel, over the last 1,800 years, the Torah center moved from exile to exile usually based upon external factors, mainly the economy and anti-Semitism. It started in Babylonia, moved to North Africa, then to Germany-France, Spain, Eastern Europe, and now, once again, we have returned to our final and original destination, back in Israel. There were also always vibrant Torah communities in Turkey, Yemen and elsewhere, but they were much smaller and less connected with the mass of the Jewish population (for example, 100 years ago, there were about 9 million Jews in Europe, and only about 1 million in Africa, Asia, and the Ottoman Empire combined!). The strong Chassidic movement in the 17-20th century Ukraine was inevitable, for hassidut was geared to the less-learned working class, that they could and should feel close to God, despite their lack of Torah scholarship. This was the situation regarding a large percentage of Ukrainian Jews at the time. Even then, the Ukraine was never the Torah center (as measured by top yeshivot and historic halachic authorities), which at that time was centered in Lithuania, but rather a bastion for Chassidic tzadikim.