rDNA is repeated many many times and is located at several places in the genome. Presumably we need so many ribosomes that we need to have many copies of it or else transcribing it into rRNA would be the bottle neck in translating all mRNA. Can't have any proteins without rRNA!

Two guesses. rDNA folds itself so much that it becomes hard to read. Using the ribosomes to read ribosome dna might cause the ribosomes to bind up.