REASONS FOR VIROID ACCUMULATION. “The reasons for transposable element build-up probably involve several processes. Elements that would normally be eliminated by ectopic exchange may fail to be eliminated from the neo-Y, due to lack of recombination. The efficacy of selective removal will also be reduced because of low recombination rates.”
http://www.nature.com/hdy/journal/v95/n2/full/6800697a.html