Because that doesn't lead to lots of repeat business as users wouldn't have to keep buying new lenses.

I think you can find out from Sigma UK by the lens serial number if it is compatible with newer Canon DSLR. For example if I recall with the 135-400 the non compatible lens started with a 2 and the compatible started with a 3.

As a general of thumb any Sigma lens older than the Canon 300D is unlikely to work on the current Canon DSLR range.

Thanks Rob, I will bear that in mind about a lenses older than 300D thats a good one to remember.

Don't despair Christine. It won't work except fully open, but you can exchange it for a new Sigma at cost price; if its cheap enough it may be worth it