Another thing to keep in mind that there is a finite number of demons, since they are the fallen angels. On the other hand, the number of people on earth in the time of Christ are estimated to be anywhere from 200 to 300 million, while it is now approximately 7.3 billion (~7,293,000,000). So if we use the 300 million figure, our current population would dilute the demon population by a bit more than 24 times. As a consequence, mathematical averages would mean that demons would appear only about 1/24th the frequency. If the population in Jesus time was 200 million, today they would appear only about 1/36th the frequency. I think that would explain much of it. Also, as they are exorcised over history, do they just go to another body or do they go to Hell; if it is Hell, then the number of demons on earth since the time of Christ have been declining, too.
Your brother in Christ,
Bill Velek