It is because (1) it will be expensive to replace, (2) it is probably the result of some other underlying problem, and (3) additional repairs may be required in addition to replacing the head gasket itself.
The head gasket seals the cylinder head to the block. When it blows, it may allow coolant to leak into the engine's cylinders and/or allow compression to be lost between adjacent cylinders or to the outside.
Head gaskets typically fail at the weakest point, which is the thinnest area between adjacent cylinders. The gasket cracks or burns through, allowing compression to leak between the cylinders. This causes a drastic drop in power that is hard to ignore. The engine may still run, but very poorly.
If the gasket leaks coolant, it may cause the engine to overheat. Coolant in the crankcase or cylinder is also not a good thing to have because it dilutes the oil and accelerates wear. If enough coolant leaks into a cylinder, it may even "hydrolock" the engine preventing it from turning over when you attempt to start it.
Conditions such as engine overheating and abnormal combustion (detonation and/or preignition) can also cause a head gasket to fail.
Improper installation may be another factor. If the person who installed the gasket did not torque the head bolts in the proper sequence or to the specified value, it can cause the gasket to leak and fail. Also, if the head and block were not clean, flat and finished properly (too rough a surface finish) when the gasket was originally installed, it could contribute to a leakage problem.
Some engines also have a reputation for eating head gaskets. The design of the head along with the location, size and load on the head bolts has a lot to do with how long the head gasket will keep its seal. In some of these engines, the problem is the design of the engine while in others it is the design of the head gasket.
To replace a blown head gasket, the cylinder head must be removed from the engine. This can be a major undertaking depending on the type of engine and how much other stuff has to be removed to pull the head.
Once the head is off, the mating surfaces of both the head and block should be inspected for flatness as well as any damage that might have contributed to the gasket's failure or might prevent a new gasket from sealing properly. If the head is warped, it will not seal the new gasket properly and sooner or later it will again fail. Resurfacing and/or straightening the head may be necessary. An inspection of the head may also reveal cracks or other damage that will have to be repaired before it can go back on your engine.
You should know that several gasket manufacturers offer special "problem solving" head gaskets for certain engines that have a high incidence of head gasket failure. These gaskets are often made of high heat materials such as graphite and have other special features such as additional combustion chamber armor, added sealing beads, more thickness, etc.