This has actually been covered many times, though we'd all like more detail on it. But, short version is pretty well summed up by:
The TB uses a new approach to keyboards. MultiTouch keycaps that can sense where you're fingers hit, how hard, etc. With multiple characters on each keycap (3-6 main characters), it is really very complicated for the firmware to get it correct. And then there are the use of magnets to determine much of the key feel, processing to judge the environment many times a second (I think) because that affects lots of other things. Complications with BT-LE which apparently is trickier than the older BT. Etc.
But the fact is, new technology is far more likely to have unexpected problems than creating something "new" but which actually is just a variation within old tech.
So, yep, everyone gets frustrated with the many delays. It sure seems like it is "too" many, but that's the way it is.
I suspect many companies would have shut down by now. But WT has said it is doing it's own funding and so I guess they have some deep pockets. Good thing because the TB really is a nice device.