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GCs are extra processing effort. They are unlikely to be used in an RTOS or an OS with tight-scheduling. OSes are usually designed to be "lean and mean" ( except windows which is just mean :- ).Garbage collectors condition programmers to not even think about these things. This saves some time, but I'm not sure it's always beneficial for the overall design of the code.
GCs are justified for languages such as Lisp which strive for high abstraction.