The bob travels in an arc, i.e. there is circular motion.
Hence, for the bob not to continue moving at a tangent to its path, there must be acceleration towards the center of the setup.
Think of just the vertical components. If the weight = tension, then the bob would stay at a constant height, as there is no resultant force up or down.
Since the bob does move up, however, there is acceleration upwards and therefore the tension is greater than the weight.