Some engines are more popular than others. They are not as easy to find when someone needs them. Some people prefer to purchase used ones because they are much cheaper than getting one that is brand new from the factory. A remanufactured engine might also be cheaper than a new one.
Sometimes, when people buy a used engine, they have to rebuild it anyways. This can take a lot of time and money to do this the right way. Something that has a lot of miles on it is not going to be worth a lot of money. A remanufactured one has basically been rebuilt but they also went through it and anything that showed wear was replaced with a brand new part.
These are also going to be considerably cheaper than a new one. They are probably going to be more expensive than buying a used one that is all originally throughout it though. Figuring the cost of the motor and the rebuild, someone may be better off getting a remanufactured one.
The engine is a very important part of any vehicle. Without it, it is not going to be useful. Some of them will have different options available for a certain type of engine but not all of them. Some have to be specific on what it is. A mechanic would be able to look it up and see if any other year or models can be used with a vehicle. If not, there may be a big shortage of engines.
More info: Remanufactured Engines