Main reason--> Polls showed Bloomberg getting crushed in a 3-way race.
Bloomberg also didn't want to hand the election to Trump or Cruz according to the article.
Former New York City Mayor Michael Bloomberg said on Monday he will not enter the race for U.S. president in November, because it was clear he could not win and he did not want to risk enabling a victory by Republicans Donald Trump or Ted Cruz.
Last month the Financial Times quoted Bloomberg as saying he found the campaign discussions "distressingly banal" and considering running for president.
Bloomberg said in a online column that in a three-way race, no candidate was likely to win a majority of electoral votes, handing the choice to the Congress.
Here's a Fox News poll released in February on a 3-way race---
Donald Trump vs. Hillary Clinton vs. Michael Bloomberg
via Fox News