Essentially it's down to the aggregate used to make the concrete and the nature of the material itself. Concrete is approx. sand or other coarse rock/stone that has been "glued" together with cement. Under compression the unilateral nature of the binding agent (cement) means that the compressive energy is efficiently distributed. However when put under tension faults and fractures will quickly develop due to the uneven distribution of tensional energy across a plane rather than across the whole mass.