giffgaff can't match every single tarriff ever invented.
If you prefer the ASDA deal get an ASDA sim and buy it then.
Giffgaff offers what it does, unlimited everything for £30 including alot of things the asda deal doesnt include. ie free 0800 calls free giffgaff to giffgaff and free data. Also, when it says unlimited it truly means that, unlike ASDA which will have a FUP.