Why is Super Saiyan Blue (Goku) more powerful than Super Saiyan God (Goku)? I watched the series earlier. I now joined the Stack Exchange and found this forum so I searched about this but could not find an appropriate answer. Does anyone know?
I believe your question is already answered in the wikia page of Dragon Ball.

The Super Saiyan Blue is a combination of the power of Super Saiyan and Super Saiyan God. 

Or,

Super Saiyan Blue is using the power of a God {God ki} and transforming to a Super Saiyan.

Therefore, Super Saiyan Blue has much greater power than Super Saiyan God.