To be honest I don't recall answering this type of question before, not exactly as phrased in this way anyway, so I'm sort of shooting from the hip here. One thing that came to mind when I thought about the point about how things were done in Acts and such, is that the situation was somewhat different. In those cases we generally had Apostles or something close to them, the Christian community was small, and sometimes persecuted, miracles were taking place much more often, and so it generally was a different situation. As time went on and things changed, they sort of cooled, so to speak. Now the clergy might be less venerable or inspiring, persecution ended, miracles still took place but perhaps not in the same way or with the same frequency, Christianity became more popular and accepted, and so forth.
Eventually being a Christian stopped requiring a major life commitment, and all sorts of people with all sorts of levels of commitment were joining. [that is, being a Christian should be a major commitment, but more and more people might not have seen it that way] Some might not even have wanted to join, but perhaps did so for political advantages, or some other such reason. And so it continues till today. Thus it was thought best to "test" people a bit more to make sure they were joining for the right reasons. After all, in the early Church they didn't even tell you about a lot of the faith until your baptism or entrance into the Church--it was considered serious business. And still should be.
Admittedly this is overgeneralizing terribly.
In the early Church the catechumenate lasted a minimum of 3 years, if I recall correctly. Actually some put it off much longer, but that's another topic. The 6-12 months that someone might wait today doesn't seem so bad in comparison, especially considering that today the catechumens usually aren't dismissed, and have access to a ton of information. In some ways Orthodoxy loosened the restrictions outlined in the Bible, for example the passages in which Jesus talks about divorce. In other areas things became more strict, like the length of the process of being received. The bishops were given the power to bind and loose, and direct the church, after all. Anyway, that is what comes to mind right off.