It may be seized on the swingarm bearings (T-shaped collars). Use a liberal amount of penetrating oil and let it soak overnight. If you have not done it yet, remove the rear wheel, linkage, rear shock, subframe and airbox. This will give you full access to the swingarm pivot. Then, apply the penetrating oil on every mating surface across the swingarm pivot.
By the way, the right side of the frame has a bit of weld that keeps the swingarm axle from rotating. The swingarm axle head is D-shaped and engages with the weld spot. Perhaps the axle head rotated a bit and it caught on the weld. I don't it but it may be worth a look. DO NOT grind away the weld... it needs to be there for assembly purposes.
You will eventually get the axle out. Just be patient and keep us posted on the progress. When reassembling, make sure you apply anti-seize on the axle and it will be very easy the next time around.