Is the quipu way less efficient than writing? If so, are there any theories as to why the Inca stuck with it instead of developing writing? Why aren't there any other civilizations with this sort of system? Writing developed independently all over the world. Why not the quipu or something similar?
You are breaking one of the fundamental tenets of anthropology, namely to assume that all people everywhere will do like what we do.
Why didn't the Inca develop writing? Why would they develop writing? Did they have some intrinsic idea of script and made quipu instead and then stopped innovating? That's not how human innovation works. Innovations are rare (yes, writing developed independently several times around the world, but not that many times. It just spread very widely), and if something is beyond your comprehension, did you do something wrong for not thinking of it when someone else did?
There's also the issue of definition. We have looked to various scripts all over the world and called them all "writing" because we compared them to what we understand to be writing. That doesn't mean that they were the same or meant the same thing to the people who made and used them in the first place. That goes for everything. In the case of writing, yes, I would certainly call all these different scripts of the world "writing" and I cannot see them as anything else, but I am also Western through-and-through so it is difficult for me to see them as anything else. And quipu can be seen as a form of writing, just of a unique style.
But quipu wasn't the only form of record-keeping in the Andes. I don't know of any other artifacts that have been identified as having a decidedly record-keeping function, but there were many states and at least one true empire in the Andes before the Inca, and they did not have quipu (the earliest quipu are Huari, which was a territorial state ca. A.D. 800-1100ish, but contemporary states and empires like the Chimú). They must have had some form of record-keeping, perhaps something as simple as having a counting bin where one potato was represented a bag of potatoes, but no quipu.
So quipu aren't the only thing for record-keeping, and quipu are complex and able to encode a great deal of information. And why should we assume that the Inca were doing something weird or unusual by sticking with quipu for their record-keeping?