Richard Sallie's 'Replicative Homeostasis III' and the extensive references section at the end of his paper might suggest some of the reasons for this.
A little light reading for sleepness nights.
.
If I try explaining it, he might start posting here again, if only to correct my bungled explanations ;-).
You can be infected with same genotype subtype such as 1a & 1b but you can't be infected with two distinct genotypes such as 1 & 3 or 1 and 2.
If you are exposed to a different genotype the existing genotype will become the dominant genotype and eradicate the newly introduced genotype is my understanding.