Modern readers of history often wonder at apparent "regression" - how is it that we allowed Roman aqueducts to crumble? How is it possible that we invented the steam engine in Greek times but didn't actually make full use of it for more than a thousand years? The answer shows some bias in the question: what actually makes technology useful?
Technologies don't exist in a vacuum, but are interdependent to the needs of the people who are around them. For example, we had the understanding of how to build mechanical calculators (/computers) from a theoretical standpoint from the 17th century - but manufacturing tolerances were not fine enough to allow for their construction. The Greeks had at least a theoretically understanding of the steam engine, but not the industrial capacity to make it useful - the mines to bring huge quantities of metals, the amount of coal necessary to run them, etc. And engines wouldn't be that attractive anyways, when you can just buy slaves for less money!
As the Roman empire was falling, population was decreasing rapidly. Trade was breaking down and becoming more regional than international. Leadership was becoming more regional, so leaders could pull on the resources of smaller and smaller places. Now, think of what we find concrete "useful" for, even today. It's for building things that are big, and able to repeated. We don't use concrete to build a house, usually, because it's a lot easier to just make it out of wood and plaster. Edit: this is far from universal and causing some disruption - the main point is that you certainly wouldn't use concrete to build a house in Roman or Medieval times, even if it were possible, as concrete was expensive and time consuming. In Roman times, concrete was used for things like monumental architecture, large scale building of ports, etc. As the Roman Empire fell, these things became in less demand. It was more difficult for leaders in the areas that flaked off of the empire to demand large-scale participating in building projects, and to amass the funds necessary for monumental architecture on the Roman scale. Populations started relying far less on foreign trade, and became more focused on subsistence within their region.
Edit: /u/michaemoser has comment below on the importance of the decline of trade to Roman Concrete.
For concrete you first of all need cement, modern cement is Portland cement - that is produced by heating calcite and other ingredients to temperatures of 1300C. The Romans could not produce such temperatures, so they were using naturally occurring cement of volcanic origin - Pozzolana (it stinks, actually ). So you would not have that locally - you have to bring that from far away (the Romans would bring it from the town of Pozzuoli - near modern Naples). /u/michaemoser
While this doesn't describe why it was also lost on the Italian peninsula, it does speak to the importance of trade in maintaining some "advanced" technologies.
So a large part of why concrete was no longer used is simply that it was no longer useful. Passing information down from generations is an incredibly arduous task - you either have to spend many hours writing it down on extremely expensive parchment, or many hours teaching it, person to person, for generations. When a technology is no longer in demand, the effort to pass it down is unequal to the reward one would get for doing so.
We are more familiar with this process in our modern era when new processes are superseded by old ones. Will you be able to find a VCR repairman in 50 years? Probably not. But we need to remember that technology becomes non-useful for many different reasons, not just because of replacement with better technology.
Edit:
Also, there is a misconception about technology during the medieval period that it was entirely a process of decline, forgetting, etc., that was more prominent when we thought of that era as the "dark" ages. People always seem to be innovative, and the medieval period is no exception - the development of heavy plough, for instance, allowed agriculture in Northern Europe to be much more productive than it had ever been before. The medieval period had plenty of technological development - heavy ploughs, pointed arches, early firearms and siege weaponry, to name a few - but they were all relevant to medieval life.
For concrete you first of all need cement, modern cement is Portland cement - that is produced by heating calcite and other ingredients to temperatures of 1300C. The Romans could not produce such temperatures, so they were using naturally occurring cement of volcanic origin - Pozzolana (it stinks, actually ).
So you would not have that locally - you have to bring that from far away (the Romans would bring it from the town of Pozzuoli - near modern Naples).
Therefore a functioning trade over long distances(capable of moving heavy stuff) + effective money exchange would be prerequisites for doing concrete. Also concrete is a very labor intensive material - nowadays you have concrete mixers, but in Roman times you would have to mix the stuff by hand. Also you need to be an expert in order to put in the right proportion of water - otherwise it will have structural problems. Also you would have to reinforce the resulting structure by mixing in smaller stones - they did not have steel beams for reinforced concrete. The material therefore needs a lot of expertise - all this just could not be maintained.
Sources: Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World by Mark Midownik
In the thread below, /u/Aemilius_Paulus has a pretty comprehensive write-up about this:
I've got a follow-up question. I've always thought that this was a bit of myth, since I assumed that the Byzantines would have maintained this knowledge well after Rome fell.
Is this a myth or not? What about buildings built in Constantinople in the 5th and 6th centuries (such as Hagia Sophia)? Did these use the traditional "Roman" concrete?
I have a followup question that from skimming doesn't seem to be addressed - I remember reading, I think in Art History class or something that we STILL don't know exactly how they made such resiliant concrete compared to what we make today? You check out pictures of modern buildings that have been abandoned for several decades, and they're still there, but already crumbling and likely wont last the thousands of years like what still remains of the Romans. Does anyone know if that is certain or is that all false?
While it decreased in use, it didn't disappear completely. Opus Signinum was used in some Saxon buildings in south-east England during the 7th and 8th centuries.