While there were probably many factors leading up to the Balfour Declaration, the most commonly cited one is as a reward by the then British government to Chaim Weitzman for his help in their winning WW1, by developing a process enabling them to manufacture acetone, a chemical vital in the war effort. At that time Weitzman was a chemist, working in Manchester and also the acknowledged Zionist leader in the diaspora. He was appointed Israel's first President in 1948.
EU's Mogherini in Tehran to talk nuclear deal, regional issues (Reuters)
from the article: Why did Adolf Hitler hate the Jews?