My theory is that they strongly believe PCIe-over-Thunderbolt (now -over-USB-C) will obviate all need for internal expandibility/upgradability, and they went all-in on a form-factor (the garbage-can Mac) to take advantage of that... and then the market has stubbornly refused to co-operate by releasing Thunderbolt peripherals equivalent to internal components. So they're playing chicken with said market, probably under Intel's repeated promises to make it happen.