I'd guess it's because the external chassis is really expensive, and it's silly.
All the busses are there, inside the device, but nope, you have to drop several hundreds of dollars (or more) on a box that squirts them over a expensive wire because marketing and design said so.
It's true, you could add an external ThunderBolt chassis, but when the trash can Mac Pro first came out I recall that not many companies were making the chassis and they were pretty expensive maybe around $1000 from Sonnettech.com (which was added to the already very expensive Mac Pro). The expansion chassis prices have dropped now. But the expansion chassis won't allow you to add a second CPU which is very helpful when compressing video.
In a way it did. Now instead of installing the BlackMagic Decklink cards, I can buy the Blackmagic UltraStudio or Intensity and get the same function over Thunderbolt or USB3. What was once locked to one machine is usable by every machine I have, including taking it on the road and connecting to my laptop.
Apple saw where the market was going and that an iMac or MacBook Pro with Thunderbolt/USB3 peripherals can do the work that required a fully decked desktop machine 6-7yrs ago, which is great for the vast majority of creatives. But in doing so they have left a segment of the creative community without the expandability, upgradeability and speed necessary (3D, CGI and some VR, etc). But some of these were never really strongholds for Apple anyway.
As a 20+yr veteran editor who has used just about every NLE on the market, my money and time goes to FCPX unless my clients specify otherwise. It's the fastest, most versatile and most stable product out there and the only one that feels like it is truly evolving away from the original late 80s NLE paradigm. And while its use in the high end market (film, tv and commercials) in the US is minuscule, it's global footprint is growing.
The story was that for this expansion, you'd add external chassis. I'm not a video person, so I'm curious why this didn't pan out?