Thunderbolt 1/2 requires a pcie gen2 x4 connector to have enough bandwidth. The SoC in the pi4, the Broadcom BCM2711, has just a single gen2 pcie lane. 1/4th the required bandwidth for thunderbolt 1/2, and a mere 1/8th the requirement for thunderbolt 3.
To get a full 8x thunderbolt 3 connectors you need a staggering 32 pcie gen3 lanes off of the CPU. This is out of reach of all but the HEDT & enterprise platforms, to say nothing of the $5 ARM SoC chips for SBCs. Well in theory you could also use something like a Ryzen 3000 and split out the 24 PCI-E gen4 lanes into 48 gen3 lanes and then you could have your 8x thunderbolt 3 connectors, too. But that's expensive, of course.
Even though it would be limited, a Thunderbolt 3 port would expand the connectivity of the Pi, and very few, if any, devices require the maximum bandwidth to operate at all.
> I don't see any reason why the 1x gen2 lane of the pi 4 couldn't host a Thunderbolt 3 port; it would just severely bottleneck the bandwidth of tunnelled PCIe links.
But that's kind of literally the reason? An entire ecosystem of products assumes a reasonably high amount of bandwidth from the connector. That's its singular reason to exist. If you take away the bandwidth from Thunderbolt 3 it just becomes USB, and at that point why not just offer USB connectors which have even broader support and not as many cabling restrictions?
> If you take away the bandwidth from Thunderbolt 3 it just becomes USB
It becomes low bandwidth Thunderbolt / PCIe. You could still use it to attach PCIe devices which don't need a lot of bandwidth. GPUs can be attached for high performance compute where CPU-GPU bandwidth isn't critical. PCIe has non-bandwidth benefits over USB such as DMA and interrupts.
> why not just offer USB connectors which have even broader support and not as many cabling restrictions?
You can't attach PCIe devices via USB, but you can attach USB and PCIe devices via Thunderbolt.
Additionally, this would require adding more PCIe lanes to the SoC, as there isn't bandwidth to provide the two 4K HDMI outputs and the other connectivity would be severely bottlenecked.