I haven't yet looked at tornado, and it's been years since I seriously investigated twisted. That said, even though it's less code to use twisted, not counting twisted, it's going to be quite a lot more after counting twisted, most of which, as you note with "can do far more than tornado can", they didn't want in the first place.
When there's a bug in tornado as it is, they can just fix it. If there was a bug in tornado-on-twisted, and if the bug were in twisted, they'd either have to wait until the twisted maintainers fixed it, or fix it themselves. If themselves, then they end up maintaining the internals of twisted (at least for the time between fix and having the patch accepted), which means they have to keep track of and understand a lot of code that, as you note, they don't have any use for in the first place! In that situation, I might choose to write my own as well.
When there's a bug in tornado as it is, they can just fix it. If there was a bug in tornado-on-twisted, and if the bug were in twisted, they'd either have to wait until the twisted maintainers fixed it, or fix it themselves. If themselves, then they end up maintaining the internals of twisted (at least for the time between fix and having the patch accepted), which means they have to keep track of and understand a lot of code that, as you note, they don't have any use for in the first place! In that situation, I might choose to write my own as well.