Halo 5 - only the disc version goes on sale?

I’ve noticed a few times now, MS keeps putting Halo 5 on sale for $15 or $20 off, with free shipping, bit the digital download version stays full price?

On the face of it, that seems to make no sense. The disc copy and shipping have to be costing them more money already, so the margin gets even narrower with that one on sale.

I’m wondering if the sales figures were way lower than expected and they are trying to unload all the physical copies that didn’t sell?

I bought a disc copy just to keep in the wrapper. Not that it’ll be valuable later on in life, but just to have. Should have waited for the $20 disc instead of the $54 one…