It's likely because Collector's Editions are (by design) the most expensive SKU to make, which in turn means they're the most limited SKU of any game, which in turn means they are almost always the least selling SKU of any game. So if the PlayStation version of a CE is going to do low numbers, then the Xbox version of the same CE is doing even less. For example, if the last AssCreed's CE did 50,000 on PS4, it probably did 10,000 or less on Xbox One.
With that in mind, removing the disc from the Xbox version makes sense. If they can save money by removing instruction manuals, the next logical step is to remove the disc entirely. More savings, more money.
Fun fact: Collector's Editions of a bunch of other shit rarely (if ever) sell well. For example, you might have a regular/standard edition of a backpack that sells 200,000 units, and then the limited edition of that same backpack selling 20,000 units. The limited/collector's edition almost always costs more money, so retailers don't want to stock shit -- especially bulky items -- that they'll have a harder time to move. An Olympics Edition, a Championship Edition, and a Pink Ribbon Edition are some of the few limited editions that put up big numbers in comparison to the regular/non-limited product.
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