Its a combination of saving weight, saving space, and saving money.
Ultimately, in the era of cell phones, better tires, road-side assistance, and general mechanical incompetence, that little donut spare is rather pointless if you really think about it. Most will never need it. Most of those that do won't know what to do with it, and most of those who do would still call AAA, or Ford roadside assistance because it is just so much easier.
And for the small percentage who actually make it to changing it themselves, the spare is probably flat anyway, since almost no once checks the spare.