For the same condition like same density, wing area, free-stream velocity, From the above figure one can see more pressure differential and hence more lift at mid span of the wing? What causes this phenomenon? Is it due to influence of tip vortex?
The diagram might be a bit more clear if a fuselage were attached to one of the sides.
If that were the case, lift would be less on one side because of the fuselage is interfering with the air flow. Lift would be less at the other end due to washout (lowering of wing tip angle of incidence), and effects of higher pressure spilling from the bottom around the wing tip towards the lower pressure on the upper wing (vortex formation).
The airstream in the middle of the wing experiences neither of these two interferences, generating lift most efficiently.
This is why gliders favor very long (high aspect) wings, where most of the lift is like the "middle" of the diagram.