Because Judaism become stagnate in promoting monotheism evolutionary force through Jesus had to come to continue propagation of monotheism.
I assume you mean "stagnant," since "stagnate" is a verb. And I assume by "stagnant in promoting monotheism" you're referring to the Jews' failure to evangelize.
According to their own mythology (which the Christians, Muslims and other Abrahamists or "children of the Book" also adopted), God entered into a
Covenant with the Jews. This was an agreement that God would lead the Jews out of bondage in Egypt and in gratitude they would spread monotheism to all parts of the earth.
As the legend goes on, the Jews did not hold up their end of the bargain. Judaism is not an evangelical religion, and in fact Jews do not encourage conversion at all. The most prominent exception is the case of a Jewish man marrying a Gentile woman, and his parents insisting that she convert so that their children will be counted as Jews, since "Jewishness" is inherited only from one's mother.
Because of this breach of the Covenant, God (so the fairytale tells us) has been punishing the Jews ever since. This is what they mean when they call themselves "the chosen people." They have been singled out for punishment. The destruction of the Temple more than once, the conquest of their people by every empire in the region, the occupation of their country by the Romans so that it no longer existed as a homeland, the Diaspora so they no longer existed as a nation, the thousand years of antisemitism that virtually defined European Christendom, culminating in a Holocaust that killed half of them, followed by Europe's refusal to let them return to their burned-out homes and instead giving them another tribe's homeland in the middle of the desert as a place to "return" to, surrounded by their sworn enemies, and as if that's not enough, the newest threat to Judaism in the increasing number of countries where they are welcomed and treated with respect is
assimilation... all of these are just the non-stop, eternal plagues and misery that inevitably come to people who welch on a contract with God!
The Christians and the Muslims learned by the Jews' example, and their religions are extremely evangelical. As a result, through conversion, they now represent roughly half of the earth's population, whereas the numbers of the Jews only increase by reproduction.
The same way early in time, Christianity had become stagnate , therefore Islam had to come to continue expanding monotheism.
I'm not quite sure what you mean. Christianity has always had a strong evangelical nature. The Romans made it their state religion and spread it to the far corners of Europe. The Greeks went in the other direction and spread it throughout the Middle East, all the way to India.
By the Age of Discovery in the middle of the last millennium, the new stewards of what became Greco-Roman civilization continued to spread Christianity to entire new continents, making it now the dominant religion in North America, South America and Australia, a major force in Africa, and even well-established in the Far East.
Christianity has more adherents than any other religion, and Christians continue to aggressively evangelize--some sects more than others. There has been no failure to evangelize.