Mark 1:2-3 quotes from Malachi and Isaiah. Why does it only say "as written in Isaiah"?

Actually, it depends on which manuscripts you read. Some older manuscripts say "as written in Isaiah", but older doesn't always mean better. Other manuscripts say "as written in the prophets". I suspect Mark knew who he was quoting, so it seems to me that "as written in the prophets" is probably the correct reading.

But, more to the point... Prepare yourself for the Kingdom of God while still in the Wilderness of life. Don't wait til the last minute. The prophets say God's Law will be in full force in the Millennial Kingdom, so why not start keeping it now?