I can’t tell you why he did this. He is called the Morningstar of the Reformation—the first glimmer of light. Once he put the Bible into the hands of the people, England was never the same. It was William Tyndale who later translated the Bible from the original languages.
“Everyone” who was educated in Europe at that time knew Latin and Greek, not so much Hebrew. Wycliffe probably knew written Latin as well as he did written English. Actually, he probably saw more written Latin than English.