Comment by Viliam1234

I am not an expert, but seems to me it is caused by two things:

1) While quantum computers are potentially exponentially faster, they also seem to be exponentially more expensive given the number of qubits, so you actually can't save money by building a huge quantum computer. This may or may not change in future. Also, there was a problem with error correction, which is made much harder by the nature of quantum computing. Smart people are working on that, I don't know the current state of progress.

2) Despite the hype, only some problems can be calculated exponentially faster using a quantum computer, not all of them. This is analogical to parallel computing: having two CPUs instead of one will allow you to calculate some things twice as fast, but some other things will require exactly the same amount of time because their steps need to be done sequentially. Similarly, a quantum computer is like a network of billions of computers that are spread across the multiverse, but they need to all run the same code, and to compress the results of the gigantic computation into about dozen bytes. So it's great for highly parallelizable tasks where the entire required output is a "yes or no" or a single number... and less useful for everything else. That still includes some important scientific problems, such as simulating atoms and molecules. But those are not the things we typically use computers for.