Why is 18S ribosomal RNA (rRNA) used as a housekeeping gene to normalize sample-to-sample, systematic variation in qPCR assays?

18S ribosomal RNA is a widely used control for qRT-PCR analyses because of its invariant expression across tissues, cells, and experimental treatments. However, due to its extremely high expression in most cell types, it can sometimes be challenging to use 18S rRNA as an endogenous normalizer for several gene expression assays in the same reaction.