The more complicated reasons aside for the moment, thunderbolt is more or less like an external PCI express connector (with some added goodness). As a result you can access a lot of things via thunderbolt you probably wouldn't want outwardly accessible (imagine a network where anybody could do a DMA on demand). Then there are a number of other things, for example the protocol and phy not being designed for 100 meter cable runs like Ethernet, no low level networking support, the fact we have a reasonable and ubiquitous interconnect for networks already.

There are alternatives for super high speed interconnects within a data center though (think infiniband and cousins) or high bandwidth options for longer distances (like fiber) etc.