Well, if I remember correctly, the 3 that are chosen are the ones that are done at the Patriarchate... I think the expectation is that the monks will do all, but the parishes won't. I think it goes to the belief that there are different needs between the parish and monastery churches.
In this case, the 3 readings that are chosen are the best of the 13 or whatever there are... so it didn't come about by haphazard practice or Liturgical butchering, just a response to the pastoral needs. I think most of us in our zeal would like to see longer services with less (or nothing) cut - but the fact of the matter is that, pastorally, it is probably not what is in the best interests of the people. For some groups, OT readings were cut; for others, the method of chanting was changed/cut.